Have you looked into what type of results you get from doing a muffler delete with just tips?
Very common mod on 540's/M5's which have V8's which are similar to your engines.
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--'00 BMW ///M5 .:Jet BLK/BLK:. Individual - Style 65 Wheels; 9.5" Square Set up;275 Tires;10mm/3mm Spacers; Black Kidneys;Muffler Delete; 3.5" Dual Tips; Beast Power; "Dinan" Ducts; E60 SSK; BC Coils
--'00 BMW ///M5 .:Avus Blue/Grey/Black:.
'88 BMW 635CSi Blue/Tan Sport
'81 MB 500 SEC Euro Koenig
'91 Alfa Romer 164S- Red/Black/ Recaro interior