If you're only running 650's, an FP 18g will easily max them out. With a decent tune, you should be able to push near 375-400 hp at the wheels on pump gas, but you'll be approaching 100% duty cycle on the 650cc injectors.
Looking around, it seems like a lot of people do one of two things:
1) Either they choose turbos much larger than they need with not enough fuel support, meaning that they end up wasting more money than necessary and suffering more lag than they need to, which could be spent on other things, especially including proper maintainance. Too much emphasis on "max power" without considering "power under the curve". Choosing a properly sized turbo for a given power level will result in a much more drivable car. Sometimes it's not a problem, and all you'll suffer is a bit more lag and your turbo won't spool up as soon. But more importantly, larger turbochargers are usually designed to operate more efficiently at much higher boost levels than most people intend to use.
2) Alternatively they use injectors way oversized for what their turbo is capable of. This isn't that harmful, as it allows you headroom especially if you have no problems compensating for the injector size. You're better off erring on the side of too much fuel.
Too many people choose their setup simply based on blind recommendations, and do not properly size things for their goals - If you're going to rely on advice you're better off doing so with some simple estimations that will actually get you close to what you need. Without getting into the really gritty stuff, an easy rule of thumb you can use is that for every lb/min a turbo can deliver, you can reasonably and conservatively tune 9-10.5 hp out of it (1 lb/min can be roughly estimated at 14.5 cfm). For example, an FP18g can deliver ~600cfm, or 41-42 lbs/min. This translates to about 375-440 hp with a properly sized intercooler, at a boost level between 20-25 psi
As far as sizing injectors goes, RC has a great
calculator. Enter a base fuel pressure of 37 psi, .8 duty cycle and add a generous 60-80 hp of drivetrain loss (20-25% - not a very accurate number, but close enough for this kind of approximation). Use a BSFC of .50 to .55 - the .60-.65 is more typical of a *very* conservative tune like those found on factory turbo engines.
