Hey guys
ive got a MY02 bugeye, and its in excellent condition but my brother who loved the feel of mine bought himself a 95 wrx and has had backfiring problems ever since.

Ive replaced the leaking blow off valve, ive changed the radiator which had a leak, ive replaced the spark plugs which seemed to be in a mediocre condition and the car is running rich but as soon as i hit boost over 5000rpm it starts to backfire! before the sparkplugs were changed it would start to backfire as soon as i hit 3800 rpm so it has slightly gone up but still the issue is there

i dont know what other problems it could be maybe a worn turbo that needs replacing?

thought ide ask the rexperts
any help would be greatly appreciated