I've been reading your discussion on turbo's. They are fun to work with because they can build so much more boost than a roots blower. When I used to run sand drags everyone ran turbo's and you couldn't find anyone running a roots blower. As much power as I have built into my boat it can't hold a candle to the performance I had with the sand drag buggy with a turbo.
850 lb. buggy
1800 cc honda car engine totally stock
750 holley DP carb
turbo running 28 lbs. boost
0 - 80 mph in 5 sec. in 300 ft.
And I wasn't one of the fast guys.
When the turbo spooled up it was like two more engines came in, Major acceleration.
I wish now that I had gone turbo's rather
than a blower on my 540 Arias engine. At 35% over drive on a 8-71 I am only building 14lbs boost at 5,800 rpm.
As far as the water injection I ran that years ago also and it worked well but no one uses that anymore. They just richen up the secondary mixture enough to prevent detonation. If your turbos are only making 8 lbs. and you have no wastegate they are probably oversized to prevent them making too much pressure. If you go to smaller turbo's and wastegates you will build boost faster and bypass the excess.
I would think you need two carbs. I hope your turbo setup is a draw through and not a blow through as the ones that blow the pressure into the carb are very low performance.
Sorry about the rambling on.