So is this for a street-driven car? Track only car? How much are you willing to spend/change?
I can offer my opinions on things, but I think your best bet might be to see what other folks with twins are running, and what twins they are using. That will start to give you an idea of where you want to head.
IMO, when people add on turbos, more times than not they go too big. Your headers are a bit large, you'd probably want something a little smaller to keep the velocity up. I think your twins are a little large and unless they are ball bearing turbos, you'd probably get some good lag there. As far as max power, I'd suggest a single turbo; however that doesn't mean the quickest car. I personally prefer twins over a single big turbo, but technology has come a long way and ball bearings help a lot with the spool of larger single turbos. A single turbo is also cheaper and will require less plumbing typically. What are your plans for the engine management/ECU? Your bottom end sounds like it's already pretty stout so that is probably not really the limiting factor to max power you can make.
If you do decide on larger turbos for the twin setup, there's things you can do with timing (like retarding it a lot at low load/boost) that will help spool up super quick but also destroy your exhaust pipes. I wouldn't worry about turbo's limiting top speed - in reality, unless you size the turbos too small, they won't hold you back in top speed. Your cam selection is more likely to do that then the turbos. The turbos take the existing power curve of the engine, and just bigify it. I would start with determining what the max HP is that your bottom end will support, figure out what you want your target HP to be, figure out how much boost you will need to hit that target, and then start looking at compressor maps to determine which turbo(s) will get you to that pressure with the highest efficiency. Once you land on a couple possible turbos, figure out the rest of your piping/setup (intake, exhaust headers) needed to fully support those turbos.