It won't warp the head if you just tack it in front like the guy did to mine. For belt n' suspenders, tack weld it in place and then do the interference fit thing and leave just a little bit sticking out, like 1/32" for the gasket pressure to hold it in.
I doubt if anyone has actually tested it. You would have to have two Falcons or Stangs or Broncs or whatever with identical specs and run them side by side, recording the data points. You could do it first without then with and record your specs or do the 'seat of the pants test.' But you probably wouldn't really remember what it was like before, unless it was really doggy.
If you are asking whether it will make a rocket out of a brick, well, the answer is no. It may help a little. I don't even know if it 'probably' helps. It might help more than indexing the sparkplugs on an other wise stock motor. On the other hand, it probably doesn't hurt to leave it out. If you have already put your headers in, don't take them back off to put it in. If you haven't yet, then what the hey. Go for it.