I most certainly can tell the difference. When under a heavy work load, the unbalanced regulators can not keep up. Even under minimal workload, at depth, breathing normally, I have overbreathed the unbalanced regulators easily. The balanced ones I have yet to be able to overbreathe. I have had both unbalanced & balanced,.... the unbalanced regulators can not hold a candle to a balanced one.
What happens is that lower performing 1st stages experience a greater IP drop during hard inhalation, especially at depth, than higher performing 1st stages. This isn't directly related to whether or not the stage is balanced, although there is some connection as unbalanced pistons are limited in the size of the orifice.
The big thing though, is if the lower performing unbalanced 1st is paired with a lower performing unbalanced 2nd, you really feel the IP drop. For example, if you took a R190 (or similar) 2nd stage, and tested a MK2 (surprisingly good performing, but unbalanced) vs a MK25 (mega-flow rate) and breathed really hard at some depth, you'd tell a big difference. But if you did the same test with a very high performing 2nd, say a D400 or G250, you'd notice far less difference between the MK2 and MK25. I've tried this, it really is surprising.
Anyhow, getting back to the OP, I have to agree that you should have returned to the shop where the problem occurred. And actually, I find the MK25 a pretty easy 1st stage to work on, and you don't really need the bushing install tool or the bullet. (But they do make the job quicker and easier.) A MK2 would certainly fit your bill, but so would a MK5 or 10 and the tools are very similar. You do need the bullet tool for those; they have a sharp edge piston. Rebuild parts for the MK5/10 are easily available, at vintage double hose for one place.
Whatever you get, I think your best and first purchase should be the Vance Harlow book on regulator repair. That's an excellent source of information.