I wrote a big ole review of 'em, but I can't find it. Anyway, let's go:
1.The y-pipe has a 3 bolt flange to the cat, I didn't have to change it because I have a 2000 SE. IIRC, just early SVT's have a 2-bolt flange, but I'm not sure off the top of my head. The WR y-pipe does have a 3-bolt flange, yes. Me owing a 1999 svt has a 2 bolt, so i was out of luck on this one.
2.Also over a peroid of time did you get worse gas mileage better? Notice a big power gain?
Mileage suffered a little because I had a leak somewheres. The EGR does not line up correctly, so I ran without an EGR tube for a short time; during which mileage went to about a dozen mpg. Once that was stragthened out, mileage is about the same; it depends on how I drive it basically. Power gain? You betcha. Definitly a couple ponies back in the corral; I'll know for sure in April when I go to the meet in Huntsville and dyno.
As I mentioned above, the EGR does not line up easily. You can a) bend the EGR tube; which is grade 18 steel and not easily bent, or b) cut the EGR tube and install a brass hose fitting. That's how we did mine. Take off the EGR tube, cut it with a hacksaw just below the top nut - there's a straight section a few inches long; cut there. The brass fitting is used for connecting hydraulic hoses and air lines. I think it's called a "compression fitting" to be more exact. I can't remember the exact dimension of the fitting, though, and it must be exact so it won't leak.
The install was terrible, but now that I've done it, it's a breeze. If you take your time and have a spare car, it can be done very easily. If you're planning to do it yourself, a lift is very very useful. Air tools are not as useful, since the amount of space is very limited.