Have a borescope handy?
Hearing that the previous owner ran 89 octane, I'm thinking it's time to ask him how many plugs were mangled when he replaced them. I'm thinking the low octane with boost may have chipped some pistons. However before we condemn the pistons and scare you to hell, let's talk further.
Do the coils on the car look original or are they red MSD coils? MSD coils are known to cause issues at higher rpm, stock are better.
What kind of plugs did you use? Platinum are never suggested in our cars, iridium or copper are the way to go.
Do you have a scanner capable of telling you which cylinders are misfiring when you drive it? If it was a shared coil (3/6, 1/4, 2/5) then you could replace just that coil.
That's where i'd start.
