Generally you fit the 3" AMM (is that the 012?) at the same time as larger injectors - the two balance each other out, and keep things happy. Go too big on the injectors without tweaking the AMM, and it won't be happy at anything other than idle. One trick is to use an adjustable fuel pressure regulator to 'dial in' your AFR (measured with a wideband lambda sensor) at full load. The computer then has to work the rest out for itself. Full load is the important part.
First port of call...chips. That will allow more boost, then you can take it from there. I'd get a wideband fairly early on, so you can see what's going on.
We had the standard injectors mapped fine up to about 15psi (from memory) on a larger turbo (Garrett T3/T4 hybrid...so not huge, but quite a bit bigger than stock), but they were flat out at that point. That was probably somewhere around 250bhp, so 225bhp is probably a safe number for an LH-powered car (which may not allow such high duty cycles as MS). We're now running Bosch 680cc injectors, which have more than twice the flow rate of the originals.
Now for the amusing (or sad, depending on how you look at it) part...
Because we have control of the ignition timing, we have it mapped for supermarket 95 octane. 27psi of boost on that naff fuel is the record to date
cheers
James