My offy cover was pretty warped, but my machinist acquaintance had already installed rear pipe and baffles and powder coated it 'cause I had it shipped to him...so, I went ahead and used it; I used double silicone (real) gaskets and it worked fine, no leaks.
That's not the story, the story is that after it's been on there for about a year or so, I took it back off and it's a lot straighter now. Still a little off, but way straighter than it was...I could probably use a single silicone gasket now (if I had one)...the torque on it must have bent it back towards straight a good bit.
brian