There are several factors to consider:
Stable IP
The major benefit of a balanced first stage is a stable intermediate pressure (IP).
A stable IP allows the second stage to operate at the same pressure regardless of depth or tank pressure.
The forces acting on the second stage seat - balanced versus unbalanced designs
A stable IP is important as the force of the gas acting on the second stage seat are balanced by just spring pressure in an unlinked second stage, and through a combination of spring pressure and gas acting on the other side of the seat through the use of a balance chamber in a balanced second stage.
If the first stage is not balanced and the IP varies, then the unbalanced second stage has to be adjusted so that it won't free flow when the IP is at it's highest. The IP is highest with a full tank if the first stage is a piston design, and the IP is highest with a near empty tank if the first stage is a diaphragm design (although this latter case is mostly theoretical as it is so easy to balance a diaphragm first stage that no one makes an unbalanced diaphragm first stage. The resulting incase in spring pressure in the second stage increases the cracking effort (the effort needed to get gas flowing) as the IP decreases.
In contrast, if the first stage is balanced, the gas pressure acting through the balance chamber on the other side of the seat increases or decreases as the IP increases or decreases, so lighter spring pressures can be used, and the cracking effort remains much more stable as tank pressure and IP change.
Now...this is also somewhat theoretical as there still needs to be some downstream bias to ensure that the second stage will release excess pressure if the first stage seat leaks and Ip gets too high, so most balanced second stages are not perfectly balanced. (It's equally fair to point out that most balanced first stages are also not perfectly balanced either and most design will show a 5 psi or so change from full to empty tank pressures.)
Why balancing the second stage matters less if the first stage is balanced
If the first stage is balanced (with the IP staying within 5 psi or so throughout the dive), it matters a lot less whether the second stage is balanced or not as the IP is in fact stable and the pressures acting on the second stage seat are essentially constant. That means an unbalanced second stage can be tuned with the minimum spring pressure needed to balance the much more consistent IP of the balanced first stage. This means the inhalation effort won't increase as the tank pressure changes.
Other factors in second stage design matter more
There's more to second stage performance than just balanced and unbalanced, although it plays a role.
Orifice size has a large effect on flow rate - larger orifices flow more gas than smaller orifices at the same IP. However, if the second stage is unbalanced, a larger orifice means larger spring forces are needed to balance the downstream force, and that can be a limiting factor on seat life, if nothing else.
Working range also matters as a valve that just barely opens to let gas pass between the seat and orifice will flow less gas than a valve that opens farther to provide more space for gas to flow past.
Case, aspirator and air barrel design matter as well. Cracking effort is critical in terms of getting gas to start flowing, but the case and/or air barrel and/or air barrel design also effect whether the diaphragm is drawn in or pressed out while the gas is flowing. When this venturi effect is low it can prevent a regulator from free flowing if it falls out of your mouth, if it is moderate it can help the regulator feel like it is breathing more naturally and if it is excessively high it can make it feel like the regulator is force feeding you gas.
Quality also matters. Some unbalanced second stage designs have fairly generous tolerances in the parts and as such you can get one that breathes exceptionally well, and you can get one that breathes badly and just can't be tuned to the same level of performance depending how the tolerances stack. The same is true for balanced second stages as well, although most of them incorporate micro adjust features in the balance chamber and spring design so that minor differences can be adjusted out.
With all that in mind, there are some very good performing unbalanced seconds takes out there, and there are some not so great performing balanced second stages out there.
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In terms of Scubapro regulators a G260 (a balanced design) will breathe just as well on an unbalanced second stage as a balanced second stage. The limiting factor becomes the depth at which the unbalanced "flow by" design Mk 2 can deliver sufficient gas, but with that said I know technical divers who use them on helium mixes at well below 200 ft and they flow fine, and even if you factor out the better flow characteristics of helium mixes,m the Mk 2 will still perform fine to any sane air or nitrox diving depths.
In other words, I'm not fully buying the Scubapro sales pitch crap that a Mk 2 G260 would be a bad match for a diver. If it fits the diver's specific needs, it's a good match. Period.
Unfortunately, Scubapro has worked hard to package features on their first and second stages and in how they are packaged to force the buyer to move up to a more expensive first and second stage to get a feature they may want - or pay more to buy the first and second stage separately.
I'm also not in line with saying a balanced Scubapro is automatically better than an unbalanced second stage. The un balanced C series for example have some advantages over balanced designs in some circumstances.
In terms of first stages, Scubapro has always marketed itself as a piston first stage company and unfortunately, they also make what is arguably the best diaphragm first stage available, but give it second billing behind the Mk 25. The Mk 25 flows more gas, but it is just overkill can the amount that is flowed by the Mk 17 is more than adequate for any recreational or technical diving purpose. And, the Mk 17 is much more reliable in cold water, regardless of how many times they "improve" the TIS kit on the Mk 25. No surprise there - they keep improving it as it still has issues in water colder than 40 degrees and calling it the Mk 25 EVO won't solve it's inherent limitations. The fully sealed Mk 17 is also a better choice in salt water and in silty conditions as it's far less likely to develop issues due to salt and corrosion in the ambient chamber.
. greatly appreciated. in other words, having a balanced 2nd stage would have compensated for the reduction of breathing performance of having an unbalanced first stage based on my specified scenario i described above? even if the performance is lower compared to a full balanced setup, it almost negligible?
