I use a magnum primer and H-414....start about 46 grains and work up. You should be able to hit 3,600 and then some if you really want. Somewhere around 50-53 grains you might want to think it over though.

That 75 gr V-max is a real great varmint bullet and you won't do a lot better with a .257 Weatherby.

H380 or Rx15 for either the 75 VMAX/85 BT. Use regular Bob loads for starts and work up, every 'cat is it's own creature. I shoot both bullets in a Bob around 3400 fps w/ 26" of barrel, you should fare better.