my mates car did/does the same thing. only running 4psi till 5000rpm and seems to go nowhere compared to mine thats pretty much the same specs. then over 5000 the boost goes up again, had it at about 13-14psi at 6500rpm. definately goes once it has the rest of the boost into it

weird thing is it never had that problem with the 2.5 inch exhaust. it was just slugish all over and then didnt really want to go past 5000

anyway, to help you
my 1200 is running a t28 on 7psi, pod filter, front mount and 3 inch exhaust, have a walbro fuel pump running i think 36psi fuel pressure on idle. also has no idle control stuff anymore

ran perfectly fine on the mines tuned ecu i had, and runs fine on the ecu in it now
unsure on how standard the tunes are on them but ive also run a dead stock ecu and all 3 have ran fine

so id say no you dont need to tune it for a t28, it would only be if you upped the boost a lot that you would want to get it tuned for safety