you are definitly waisting the 2.5s time and urs with a vf28, you must be able to hear ir wheezing like an oap walking up a hill when she getts up to and over 5k revs?

that turbo is far too small for the 2.5 in my opinion, that vf35 seems cheap at that money but i know nothing about them so cant advise on em, tdo5 20g would be very nice if you have fueling and cooling to suit, 360 to 380ish on a 2.5 with sti heads, absolute smallest wud be an 18g, think very careful about this and try look into future, only buy the once,
all these turbos are around same price 2 to 300 quid max difference, rather than buy an 18g and try get say 350, buy a 20g and run low boost and get effortless 350bhp, and then in future if you get the mods, up her to max. i actually thought that a 20g even seemd to run out of breath up in revs with a 2.5.. thats just me do, and 20g was at her max, bless its little soul

but next jump after 20g is big one and heads on over the 400bhp, talkn your td06 and md 321 turbos then... there is so much to think of, fueling, cooling, clutch and gear box, management , and budget etc etc...
list your current spec and mods, and your budget and the lads here will be able to pin point your options