You get around 5-6 rwhp just by removing the cat and modding the airbox.
Not sure how much just the termis and ecu does as mine was broken in with it installed

But the difference between termis with cat .vs termis without cat is like day and night.
But be prepared to have it remapped also, they tend to run very lean without the cat, especially if you open up the airbox (or change to the airbox lid that comes with the Termi kit).
I got some EU dyno diagrams of my bike somewhere in these forums, got Bazzaz kit installed now too, not had it dynoed after that though, but that also made the bike a lot meaner.
All that and the smaller front sprocket really made it come to life, it wheelies my fat 236lbs behind without any questions
Edit: Oh and btw I have the termi slipons + Leo Vince cat delete instead of full termi as I like the looks of the dual underseat termis better, looks terrible with that big ass can on one side