Original Borla cat-back or a custom setup like Trubendz? The original Borla cat-back is still only 2.25" diameter piping, the same as a stock SVT. You'll have slightly better flowing mufflers but this will not improve power more than a few HP.
While the original Borla is a great exhaust, I wouldn't recommend it for anything more than looks and sound. The performance gains (2-3 hp) are not worth the cost.