Just keep the OEM one and if you run into a problem, put it back on before taking it to the dealer. It's highly doubtful that they will ever notice you had a bolt on exhaust system installed at some point, and they'd probably never be able to prove it.
Bottom line, they have to prove whatever modification you made was the direct cause of the malfunction. I doubt very much that they could ever do that with an exhaust system, if your engine just happens to "mess up."
'11 BMW E92 ///M3 - ZCP and DCT
'15 Ford F-250 - Lariat, 6.7 Powerstroke Turbo-diesel