BHP increase on NISSAN SILVIA SPEC-R AERO (S15) ‘02

[nexus69]

What is the max BHP increase on this car ?

Also if i increase by say 15bhp then do an engine re-build will it go back to the original bhp or stay at the 15 extra bhp ?

What is the best way to maintain the extra BHP and also is the gained BHP worth more in power than a part upgrade like racing airfilter etc ?

Thanks.