I really liked the sound of the stock exhaust (none). If I could get a 3" exhaust made that had that little sound, good flow, and nice flared tips, I would definitely use that. The SPT sounds and looks great, so I'm very happy with it even though it's a bit louder than stock.
I'm looking forward to seeing that 50-100mph pull. It makes sense to wait on E85 and bigger power until you swap the block. I probably would have just done it all at once though. Save on labor and have a mint OEM shortblock to sell. How do you like the 5D3?