compared to one that might have a few small creases in it?

 

Mandrel over crush bent, I dont think you would notice too big of a difference. Now if its that kinda bend where it actually creases (ripples) the inside of the bends, then you might suffer a bit. Most people generally oversize their y-pipe anyway (dual 3" into single 3"), so to be honest with you, a normal crush bent y that just decreases diameter slightly in the bends would be just fine.

 

Unfortunately, I don't know the exact equivalent. . .

But a crush bent 3" pipe flows only as good (if not worse than) a 2.5" mandrel bent piece.

 

Ok,
bends less than 30 degrees won't affect the flow too much.

But, here's a page that shows the flow difference.

http://www.a-1performance.com/performance/

so, a press or crush bent can be expected to flow about 80% of a mandrel bent exhaust.

I think even that might be generous.