I have a 98 coupe (i4) and have used nothing but chevron supreme with techron from day 1 (since I have owned it anyway) and never noticed any problems---nor have I noticed any performance increase...

It is hard to say without having ever used regular to compare. I would seriously doubt a 5hp gain from fuel alone--unless maybe you're comparing regular to 76 cool blue or something--but I don't know for sure.

BTW pm me if you are interested in getting rid of your underbody kit