Jay,
I saw no HP gains. Didn't dyno before and after install, however. Most others will agree with me. Probably see a little EGT decrease however. Biggest benefit is no warpage like the stock one does, with the possibility of breaking the bolt "ears" on the head. I suspect it flows better, too. Just compare them and you'll see.
As for pre-assembly, unknown. However, an hour or so prior to install I put the center section (that has the "female" ends, right?) in the oven at about 200* and the end pieces (the go inside, right?) in the freezer. When I was ready, I pulled them out of their respective boxes and slipped them together. Piece-o-cake. However, you better have some way of making sure the ends are properly spaced to match the bolt holes on the head right off because in a few minutes, they will equalize and get tight. I didn't have them exact and had to fight it for a while. Maybe a 2X4 with some holes with wooden pegs driven in that match the head.
It will be interesting to hear what they have to say.
Regards.
The other Jay.