Help needed with spark plugs
 

In need of some help with spark plugs.
I currently have a subaru sti 04 fully forged
With md321 turbo and other mods, the car is running aroud 450bhp
On last dyno, but im looking at changing the spark plugs which would be better
Heat grade 7 or heat grade 8. I currently have heat grade 7 but have noticed they seem a little fouled up and if im driving the car when hot it on the odd occasion will stall then takes a while to start, so any info would be grate, many thanks