Oh sorry, I misread that you already have the SLP. It will be even easier with that.
I like the Magnaflow a lot. Sometimes I have wished it were a little louder, but it's really loud enough at WOT even with the cats in the X-pipe. If the SLP Loudmouth is too raspy for you, I think you will enjoy the tone that the Magnaflow gives.
Just remember that the Magnaflow will get a little deeper and louder sounding with time and miles since the packing inside the muffler "loosens up" a little after the initial burn in.
My only complaint about the Magnaflow is the wimpy looking tailpipe tips. If you don't resell your SLP, you could probably reuse it's tips on the Magnaflow kit. Otherwise, you might want to order the MRT exhaust tips with rolled edges.