[safe]

I have the 21/27 combination, It's a street set up.
I do some auto-x and track days and if I had done it today, knowing what I know, I would have gone for 21/28 or 22/29, not sacrificing to much.
I would defiantly stay away from 22/27. I have a set of 29 on the shelf, thinking about installing them for 21/29, but I don't have the time or energy for it right now.

The 911 is relatively softly sprung and, in general, you need to double the spring rate to notice an immediate improvement. To get springrates that are in the "race car neighbourhood" you need to put in the biggest Tbars that fit or bigger (coil overs).