Sorry, this got pretty long,
I guess I can chime in a bit with my turbo experience. I have to agree on the stand alone EFI system. From all the people I know that have tried to add a turbo on the cheap, anything over 6 psi of boost is just asking for trouble with a piggy back system. I ran an Electromotive TEC II with 550 cc injectors (rated at 45 psi, I was running 52 psi fuel pressure) on a 4 banger (Toyota 22RE in my Celica) an it was running close to 90% duty to make about 300 hp give or take. This suggests 8 of them would handle 600 crank HP. That is abit overkill for your goal, but not a problem with a stand alone EFI system. It would probably run like crap out of boost with a piggy back though. The only piggy back I really tried to tune for a guy was nearly impossible. Since it was a MAP sensor honda system we were able to use a rising rate FPR with 320 cc injectors (I think 280's were stock in the ZC motor) It would go closed loop at idle, but just barely, sometimes in different weather it would throw a rich exhaust error code. The rising rate regulator pushed the fuel pressure up to 90 psi by 8 psi of boost. I made sure the leanest it got under boost was 12 to 1 on an FJO wideband O2 meter. We used most of the range on an SAFC box to lean it down out of boost, and richen it up in boost. The 2 bar GM map sensor was not a good fit for the stock Honda ECU. But it did run strong and lived until the owner felt he could tune better (without a wide band O2 either)and went back to the 280 injectors because it idled too rich and got only 18 mpg. He made it 40 miles out of town before he blew the head gasket. After he replaced the gasket, he made it another 100 miles before he cracked 3 pistons. He tried to blame me, but I saw the 280 injectors in the motor and the 320s in his tool cart. The other Honda I did at the same time had a TEC I setup and we put 450 cc injectors in that one and made 225 hp out of a single cam 12 valve 1.5L in an 1800 pound 1985 Civic. That one lived until the drive caught 1st while going for 3rd, OUCH. Pushed all 4 exhaust valves through the pistons and trashed the head. Data log showed over 10,000 rpm.
Single vs Twin turbo??
Properly sized, either will make fine power. Lag is not going to be much different if they are both sized for equal air flow. The total spinning mass is more with twins, but the diameter is smaller, so the polar moment might end up a little less, but it is a tiny difference. I fel the greater reduction of lag in most twin setups is from having very short exhasy paths from the cylinders, and being able to have the pulses go right into the turbine housings. Using a split housing single on an inline 6 probably spools faster than twins. My friends turbo 355 Chevy is a single split housing with the firing order pulses split evenly from side to side on the turbo. It is alot of plumbing to do this, and it is not even close to equal length, but it is making 20 psi of boost at under 1500 rpm. It only dynoed at 600 hp, but that was at just 4200 rpm, it was making well over 800 lb ft of torque at even lower revs.
If it was legal for me to turbo (SCCA Street Mod class, no boost over 3.0 L) I would go with a single split turbine housing and make up a flap to force all the exhaust to one side of the housing until it hit 6 psi of boost. It would be basically 2 wastegates, one opening the other half of the turbo, and one venting to control max boost, arounf 15 psi or so. Mazda did something like this on the 88 RX-7's.
With twin turbos running into one throttle, I al always concerned that one turbo will end up doing more work. If the wastegates are not perfectly balanced, the one that cracks at a lower boost will start slowing and the other turbo starts to feed the whole motor. If there is no balance pipe ahead of the turboes, then this could go all the way to where one turbo is producing all of the boost.
Gary M.