With my 2.5" downpipe,hai,hv3, 91 octane overkill incar tune I took my buddies bonne ssei by a full car from 100km/h to 180km/h. But they are heavier then a gtp.
I have a complete topswap that I was going to install but backed out, I can't afford a breakdown if for some reason something went wrong so I'm selling it and will get probably rockers one day.