I remember years back being at my friend's Uncles shop. One of his other friends was there with his 69' Chevelle (with a mild rebuilt 305) getting it tuned for a couple hours to race it that night.

We all went out there, and as soon as he got to the pits he was ripping of the exhaust. I asked my friend "isn't that going to throw off all that tuning they just spent half a day doing?" He just shrugged his shoulders.

Another story, two youngens in a early Nova. I was running my '93 Z28 that day (13.8s). At the track, the Nova guys uncorked the exhaust, and mounted slicks (I ran same street tires that got me there). They clicked off mid 14's. Not bad, but I'm guessing the exhaust was not holding them back (if anything it probably would've ran faster).

On the way home, they modified the 4 on their 14.50 dial in, to look like a 1. That kind of irritated me, so I got next to them and bugged them for a mile or so trying to get them to back it up (young and stupid). He wouldn't even look at me (probably remembered how fast my car was well)!

Last one
I had my converter on my exhaust (same car) modified so that I could replace it with a straight pipe. Clicked off the usual times for a bit and then dropped the cat. Before installing the pipe, I just couldn't help but run it once with open exhaust. Without the backpressure it was loud and idled like a pro-stocker, but lost a couple tenths (which I figured). Put the pipe in and ran the same times as with the cat! I think I gave up some torque for top end power.