For anyone that doesnt know, Im one of the guys who ran an 8.6:1 n/a longblock in my GT. I ran a stock turbo with all GT stuff (since it was in a GT...) and it ran awesome. It's still running to this day, 30k miles on the n/a block with a stock turbo with no coolant.
When I had it, I ran 10 psi (basically disconnected wastegate, can't get any higher boost at this elevation) and an 85 shot nitrous. I used to be able to hang with my buddies 345 whp 300zx TT til like 65
The guy who owns it now abuses it badly, in fact he even towed a 3/4 ton chevy truck with it on the interstate... It is still running awesome, though it burns a bit of oil.
Lower compression will indeed help avoid detonation. For the era of the 1G GT's.
Mazda was thinking ahead knowing there'd be people who put 85 octane in and drive in full boost on a 110 degree day
With todays better combustion chamber designs and advanced electronics and knock control devices, it is possible to run higher compression. The head design of the F2t isnt near what new DOHC 4 valve/cyl designs offer. The slant plug and 12V chamber is more prone to detonation from what I've observed in both my F2 and my buddies KA24E (12V SOHC 2.4L) Those engines had 8:1 n/a some years! I see a correlation there, but Im no engine designer. Hell, none of us here are.