Has anyone used one? How was it?

Really not needed. You want to keep the hammer spring strong. I changed mine once a year when shooting IHMSA because accuracy would drop off. I now use the Wolfe over power springs.
The SBH trigger can be taken down to 2# easy with a little grinding and honing. Mine is 1-1/2# but I had to make a new transfer bar to cover more of the firing pin. You can get in trouble if the trigger kicks your finger forward using a stock transfer bar.
You can improve it by unhooking one leg of the trigger spring.
Never reduce the mainsprings! If you do, you need a very fast spring to make up the difference. A lighter hammer would help.
You MUST maintain proper ignition.

Is the mainspring the hammer spring?

The kit has a bisley style hammer that I assume is lower than the BH hammer as well as the trigger and springs for both. I ran across them while looking for the Wolf spring kits and thought I might like the hammer. If I were to get brave and do some honing, is there some information that would show me where to sand or not to sand?