^^^^
A Cat Back will bring no gains as the restrictions are still in place (in the downpipe), you'll just be adding noise. Changing thee hole system inc the downpipe to a 3" or 3.5" with a low cell cat (either 100 or 200 cell) or a decat will aid flow and thus aid increasing the BHP. Low (or no) restriction downpipe is a supporting mod IMHO which help along with other "flow" mods...CAI and IC and that's where all the gains will be found.
As for how many ponnies are added by the addition of solely an aftermarket TBE.....not sure.