Personally, I put a ZZP 2.5" downpipe on my stock 99GTP engine and did a tune up, (plugs and wires), and ran 93 octane and I saw 0° of knock except ocassional 6.9° blips for about 0.6 seconds at WOT when it would shift. But every engine is different.
Unlucky for me, however, I think the car was sorta chipped when I bought it but had a lot of power. I found out it had a lean issue and about 11k miles after I bought it my engine blew. Went through spark plugs like no other o.O
May not be a bad idea to rent or buy a tool/camera to look at your pistons with. I bought a little camera/endoscope off of Amazon for like $20 that I saw my piston's problems with. Althought probably not necessary, however may be worth it to keep the idea around
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