Decrease in power

Hello ... i have a mitsibishi evo 2 ... i have
recently picked it up from the shop after a
bottom end re-build ... the top end is alright
they say ... the exhaust is a straight pipe ...
the wastegate is big ... racepro twin trumpet
bov ... ive recently ran it in and i am at
800kms so far ... ive opened it up here and
there a bit ... (not fully thrashing ) all has
been well but tonight i have noticed a
decrease in throttle response or something
... i dont know if its got something to do with
airflow or something ...it dosent want to get
up there as easy as before ...like its staining
a bit ...please help