You will find that the stock seal has an O.D. of 2", 50.8mm so this unit above will have an inlet too small to suit the TTS tail pipe.
You could cut off that inlet and weld a larger I.D. tube to it.
Remember tho' "a chain is only as strong as it's weakest link" and a pipe will only flow as much as it's smallest diameter.
So you need to maintain that header tail pipe diameter right through the system, or at least up to the point that it splits into two (or three) pipes, mufflers/Silencers.