how much horse power will it be?

It depends on what kits you go with. The exhaust kits I used on my v6 (granted it was about 10 years old than yours) gave me something like 65 more horses to play with but they werent necessarily top-notch kits so do some research online and see what is available for your particular engine.

An exhaust system will give you maybe 8-10 hp. 65 hp is being a bit overly enthusiastic. A head swap might give you that.

oops my bad I meant both the exhaust and intake kits and that was just an estimate since I didnt have any way of actually testing the gain I recieved.

your starting with 210 and your maybe (if your lucky) putting 175hp to the wheels. The exhaust (full true dual im assuming) and CAI with a tune will net you maybe 25 at the wheels if your lucky. Id rather you be dissapointed than misinformed.

Alex is correct. Exhaust (even true dual) and intake won't give you that much. I had an 02 V6 and added a Steeda CAI with a true dual Borla exhaust set-up, so not your bottom rung kit, and it only got me about 20-25hp at most. I added a supercharger and that was much more effective. Your typical bolt-ons won't give you very much.

How much horse power will gain a bone stock 2015 mustang v6 with the addition of a borla attack exhaust?