I would have to say turbo :D
I started thinking about fitting a Turbo to my IS last year........and i asked alot before i went and done it! ;)
I did not go out and get a Turbo kit for my IS, i made the Turbo Kit up! buy doing this i could pick the bits that i wanted for my turbo "ie" HKS BOV, JDM/wastegate, Greddy E-manage and so on.........
The only thing that was a bit over sized was the Turbo i picked, but i new this..........untill i get "forged internals" and lower compression ratio, then and only then will this Turbo be at its best, only time will tell.........this is all being done over the winter months :winky:
when i fitted my Turbo i had it on a rolling road 3 weeks later and at .4 boost i had 198 RWHP .............that was back may!.....but i have had some ups & downs with it as i have not had it maped.... ;)
But the meet i went to at janspeed, rolling road day! i was boosting .55 and i only got 152 RWHP so some thing is up some where.......all we know is the fueling needs to be upgrade as the injectors start to max out at .4/5 + i need to get it maped (yes i know it should be done)
I will be doing alot to the car over the comeing months, to try and get it all done for the 1st Jap/meet next year :D
so as to your Turbo is the only way to go