This is in the regulator forum rather than the Scubapro forum so I'll be really honest and not feel too guilty doing it, but there are a few things to consider about "upgrading" an already very good regulator...
The Mk 17 is a sealed first stage and uses a thinner diaphragm and an extra washer to give the poppet a bit more working range. Which is to say it offers a little more potential flow than the MK 11 (although Scubapro could do the same thing to the Mk 11, they just don't) and if your Mk 16 has been upgraded to Mk 17 internals (which is almost certain if you've had it serviced in the last few years as Scubapro did the upgrade free at annual service), the Mk 17 is a better match to the Mk 16 than is the Mk 11. And the better cold water performance is extra frosting on the cake.
The C200 and C300 are cutting edge in terms of un balanced second stages and offer very good performance, but I have not found mine to be on par with an S555, S600 or G250V, and my opinion is that any of those three would give you performance closer to your current S550.
Personally, the Mk 16 is a superb reg, and the upgrade to Mk 17 internals resolved any seat alignment issues that may have remained in the Mk 16 design. (Scubapro has historically used an evolutionary design approach and makes upgrades backwards compatible into older models, and that applied to the Mk 16/Mk 17 internal changes). Consequently, all you'd really gain with a Mk 17 is another HP port and a sealed ambient chamber at the cost of slightly larger size.
The Mk 11 uses the same basic reg body as the Mk 17 (in fact, I added the half a dozen or so different Mk 17 parts to upgrade the two Mk 11s I had to Mk 17 status) but lacks a sealed ambient chamber so it is a little smaller. As noted above, is "detuned" a bit compared to the Mk 16/Mk 17 in terms of maximum flow potential, so you would not gain anything going with a Mk 11 other than slightly smaller size than a Mk 17 and minimally smaller size than a Mk 16.
Thus if it were me, I'd stay with the Mk 16, unless I needed a sealed first stage for cold water diving, and/or a second HP port and just get a new second stage if I felt the need to upgrade anything. I'm not sure what you are using for an Octo, but your "old" S550 would be great in that application if you get a new S600 or G250V second stage to go with your "old" Mk 16.
Another thing to consider is that Scubapro dropped the free parts for life warranty, unless you buy the reg as part of a reg/BC/computer combo, and that adds up to a LOT of money. Which means if you have kept the old warranty up with the required annual services (or rehabilitated prior to November), you'll continue to get free parts on your old reg, that you can't get on the new reg unless you go with a whole new reg/BC/computer package. Some dealers will "batch" sales of a BC, reg and computer among different customers to get the FPFL for the reg customer, but that gets complicated and many dealers probably won't play that game.
So until Scubapro gets a clue and goes back to the old FPFL policy, they've actually created a disincentive for customers who got regular annual regulator service to buy a new reg.
