Damn those are some very low numbers! (oops, guess you've changed your story already!)
$1000 for a mandrel bent exhaust...
Boy are you confused...
I built a mandrel bent exhaust for under $300.
$600 for high quality stainless units.
Borla dropping power???
Even though there are numerous dyno's showing gains every time!
4.2" pipe??? huh?
Do you mean that crush bending a 2.5" pipe at 90 degrees effectively makes it a 2.1" pipe (stock is 2.25" mandrel BTW) That's a power loss in any book. Not including the added weight of running 2 sets of larger pipe all the way to the back (a good 20lbs added or 3HP/TQ lost due to weight!)
Without testing the same car on the same dyno with these different setup your comparrisons are moot. The 177HP is a good showing, but I will openly bet you it would be higher with a better system (which would be anything - even stock sans resonator!)
Plus dropping that extra weight off the car would make it quicker, faster, and handle better!
