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I actually run this setup on my 535.
Stock exhaust with 200 CPSI catted RM downpipe (ebay)
Sound increase is minimal. While cruising, I would say same as stock. When driving aggressively turbo whistle is more prevalent. I used to have a stock dp with M-performance exhaust, which caused excessive drone and why I went back to stock mufflers. I have a 6MT, so i think drone is more pronounced because of different (from 8at) cruise RPMs and using larger throttle inputs in higher gears (w/o downshifting). But now with hi-flow catted dp and stock exhaust no issues at all.
I am using MHD stage1 with catted dp and upgraded intercooler. I was planning to run stage 2or 2+, but these mods improved the power delivery and spool enough, that I am sticking to stage1 for now.
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