I have this header. It is a 2 piece slip fit design and it is a PITA to separate, but I was able to get it apart with some help from my brother. I held the top piece and he hammered off the bottom piece with a mallet (we work in construction so we're used to this type of BS). No damage was done to the header so I would say it's pretty strong.
When it's separated it's pretty easy to install. If you were to install the header as a single piece I think you have to loosen the engine mounts and jack up the engine a bit, which can be a PITA. I was able to get the top half of the header in from under the car. Then I attached the bottom piece and lightly hammered it onto the top half just enough to get the screw/bolt that holds it together on. Then I tightened the screws to bring them together.
The quality is good especially for the price, I would not have bought this header if the price was any higher due to the fact that the stock header is already pretty good.
As for the performance, the jury is still out on that. When I installed the header the exhaust note definitely changed. I have a HFC and the tanabe. The tanabe is boomy and droney at times and after I installed the header most of that went away. I think it has to do with flow of my setup. The entire exhaust setup is quieter under normal driving conditions and at WOT it comes alive. I'm currently getting a tune done for the track and it looks like I have some gains. Hope this helps, let me know if you guys have any other questions.