There is only so much retard available from the knock box and det sensor. If you use a stock CR of 8.7 for a carb block, and 7.0 for the turbo motor, that's an increase of almost 25% in CR.

If you look at it another way (and somewhat simplisticly), at 7psi of boost and it's effect on effective CR, that's about 1.5 x 8.7, or 13.5ish to 1, and you're past the detonstion limit of pump gas, so it takes a hammer to the pistons and ring lands, regardless of the amount of spark retard you throw at it.

Youjust might get away with that CR on a better breathing 4 valve head like a 4G63T motor with a better pent roof combustions chamber, but the 2.6 just won't take it without some addons, like water injection.