Our race vehicles are subject to a 95Dba 30 foot drive by test with car under full laod.
My current race car (not Z34) is a 5-litre V8 with 14.5:1 compression pistons on E85 and carbon airbox that breathes thru a AMG C63 filter ..... so induction noise (which is a significant contributor to noise on the car) is about as "muffled" as it can be
I use SpinTech race mufflers - 1 either side and consistently run 94.3 Dba, so I suggest you might like to investigate these mufflers for your car.
www.spintechmufflers.com
My current race car engine uses a pair of 1441's and I have a pair of 3331XL for my new Z34 engine (will be 9000rpm capable, hi-comp on E85).