few questions about tuning a 01 wrx bugeye

this is a 01 uk wrx bugeye that has a downpipe and sports cat and mapped by power engineering. ecuTEK we think.
he is looking to get some more power from it.

what kinda power can wrx engine take?
what power is his standard turbo rated for?
injectors are good for?

would this be a good way to go.
1/ a 99 sti type r turbo, sti5 tmic, sti5 injectors and a remap.
or
2/ his turbo..01 sti injectors...sti5 tmic and remap.

what kinda power would he get from each of these ways?

3: if on a budget

used sti8 tmic, used v34/35 turbo, walbro pump. fuel reg,

what kinda power would that get him.

no avcs on wrx so maybe around 310-325
type power.

also wat fuel going to be running

he is using e5.
i think he has a rolling road print out from the last owner saying its 310bhp.
but from the mods that are done to it, i doubt it.
ecutek maybe.
decat downpipe,
sports centre cat and no idea what the exhaust is but it has a tiny pipe. a fiesta would have a bigger one,
the owner before hom said the exhaust cost 3 grand
fuel pump maybe.
thats all id say is done to it.