Hey mate i had the exact same problem with mine, was a nice smooth drive until i planted it, was alright at the lower rev range then wen it hit boost it was pop pop pop out of me exhaust, had stock boost, regapped the sparks, cant remember what else i did but nothing helped, if you have the stock coilpacks ide be getting some new splitfires!
Put the coil packs in and runs spot on again.
Yeh i agree dont pay much more than $500. for the coils and installation, if you shop around you'll find it for that price if not cheaper.
Good luck mate
James