I've had the Quaife in my car for about 43,000 miles and I still think it is the best money I have spent on the car.
At the limit you can feel a bit more load on the steering wheel, just the tires trying to straighten them self out. You can induce a lot of understeer if you are not carefull, if you charge the corner and just toss it in and stand on the gas it is going to push (a lot of people like a big rear sway bar and the stock front bar to help this condition) My guess is it's worth about 3.5 seconds a lap at the race track (road course) as for what it does for your 1/4 mile times you will have to ask someone else, I have NEVER run any car in the 1/4 mile, but I do have over 10,000 miles on my BMW at the local road course and 6 200 mile + days in the SVTF so far.
BTW if you are thinking about putting one in your car (either the Quaife or the Torsen that is suposed to be coming out soon) I have written a set of instructions complete with pictures and would be happy to send them to you, Just PM me with an email address that will take a large attachment .
Nope, NA, I keep having the urges but keep fighting them. I got rid of a 89 Mustang Conv with a 5.0 and a blower when I bought the SVTF, it took me a few weeks to get over the "WHERE'S THE BEEF" everytime I got on the gas.
I have way to many toys and all of them have been played with (even the SVTF) but I don't think I am going to FI. It's fun enough for the street and a blast to thrash on guys at the track play days and when they come and ask how much boos I'm running I tell them I'm NA and they really feel like S**T. Last time out I was flashing my lights at a couple of NSX's ( I was killing them in the twisty bits and staying with them down the straight).
Just remember it is easy to drive a Fast car Slow, but a lot harder to drive a Slow Car FAST. My recomendation, get so YOU CAN DRIVE your car as fast as IT can be driven THEN start to work on making the CAR faster.