I threw some 6" race mufflers into my set up and it didn't do squat. 1db on a tester but unnoticeable to the ear. I had much better luck with some fiber glass mufflers cut in after my x pipe. Dropped it a few dbs and was noticably better on the ears.
I've been running the base 6262s for 4 years and 60k miles now. Still going strong sitting at 784whp. I doubt they are close to maxed out but your best bet is reaching out to hellion to see what the base turbos can handle.
If you enjoy racing you have nothing to lose. Send it and have fun. Maybe even get bragging rights, just be safe. I have a boosted manual gt and honestly full bolt on e85 a10s give me a better run than any m3/m4 ever have.
Race him and find out. I'd think with your mods you take him 60-130. Your set up should be in the 9s so if he's stock i think it will be close but in your favor.
Not sure if you'll ever completely get rid of this on a manual trans car with a maf regardless of the tune. My mustang is pretty minor even though it's tuned and has a solid hub clutch but my stock jeep is pretty bad. My map cars don't do it at all though so I think it mostly the air turbulence...