I've been told that a decat downpipe could cause overboost and that will make it fuel cut. I would recommend having it mapped as the car could be running rich/lean and doing damage. At 6000rpm you may be pass the maximum boost so it may taper off(using my basic knowledge).
As for the noisy reverse, is it whiney, crunchy or tappy? How would you describe it? Also does it come from front or rear?
I know my reverse is whiny but that's simply because it's reverse. Like most other cars.