How long have you had them? The reason I ask is I thought mine were too loud when I first got mine, I was like holy crap they are loud but it didn't take long for that to wear off.
With that said I think a OR/X, cat delete or Headers would tip mine over the edge of being too loud, so, if I ever headed down that path I'd probably have to swap out my mufflers.
Since you've already went down the Borla path, (I know its not cheap) the Touring muffler would be what I would be looking at doing and I dont think you will have any problems selling your S-type mufflers, there maybe somebody interested in trading.
15 F22 M235i, JB4 Stage II, Cat delete, CAI
12 GT Prem 5.0 (Sold)