I used the spring from the QF-2 kit when I tuned up my .177 Fast Deer and it did in fact give it more "zip" :)
I stayed with the leather seal as well, just reconditioned it before I put it back in.
The Fast Deers are nice shooters. I got mine when Best Airgun was selling off their stock, wish I had bought 2....
I did pick up a .22 cal Fast Deer also used from Mike Melick and it's a sweet shooter as well...
Jeff