It is very strongly advised to use forged slugs for aplications that exceed 12psi.
At a guess, I wouls say 5-8/9 ish.

Are they fun to drive???
HELL YES!!!!!!!!!

how it went

After driving it for a week while I put a cammed up A14 into my daily driver, the N/A A14 felt as gutless as hell.

The old engine s*** itself, because I was an Iidiot and gave it death when it wasn't tuned right.
My new engine I am building is an A15. I will be dispencing with the decomp shim. I may well also reduce the blower speed a bit to reduce the intake charge temp. (Although , with the new motor having a larger swept volume, the boost will come down a bit anyway.
I have decided to munch out the slugs in the lathe to lower the CR, but still leave the squish band intact to help prevent detonation.