I used this method when I first did my 3L back in 2005. The NHV was at a minimum. It seemed like it was too soft to actually help with the wheel hop situation. They didn't last very long, the window weld didn't bond to the existing rubber in the mount very well and tore away pretty quickly. It's impossible to pack the mount with your fingers without still having a significant amount of air pockets in the voids of the mount. For $20 it does something but nothing I would expect to last very long. I'd say it's a good way to repair busted mounts to perform a little better than a new stock mount would and if your drive regularly they should last a while.
96 Mystique Black On Tan V6 MTX Port Matched 01 3L, 2.5L Cams, 2.5" Exhaust Optimized y-pipe, SVT clutch, Superchips, 3L split port LIM, SVT UIM, Intake, Mustang 65mm TB, MSDS Headers. Eibach Pro Kit, Torsen Diff., Ford Racing 17" Wheels.