Just adding a data point and hopefully some useful and updated info to this older thread (rather that starting a new one).
So I too have the Borla S-Type, and I usually drive short distances around town and actually like(d) the aggressive sound. After driving on the Interstate, as others have suggested, there is definitely a Maddening (for me anyway) DRONE. Since my wife has been with me on those Interstate trips, and won't let me turn up the music...I found I had to drop to 70 MPH and then up to 90 etc...To avoid the UN-Sweet spot of ~80 MPH.
So today, I took down the front piece (Resonator) of my OEM exhaust, and removed the X-Pipe from the Borla, and was in fact, able to replace it with the OEM Resonator. It does/did require a little modification, but nothing crazy. I basically had to cut off (with a SawsAll) the expanded/flared end of the outboard/left pipe on the OEM Resonator so the Borla sleeve would fit over it.
At any rate...Just letting others know it works if you want to try it. I took it out for a drive...and NO DRONE! Also nice and quite when you want it to be, and wakes up and gets nice and loud when you push the truck hard.
I did it rather quickly this afternoon, and used the extra clamp from the OEM too...just to make a good seal...But works perfectly and no leaks.
Good luck to anyone who may want to try it. Pretty easy and straight forward. You end up cutting about ~ 3" off the OEM pipe, and if you decide to go back to X-Pipe, the sleeve is undamaged and will still work and revert back.
DC