If you already reload, the 300 whisper is not expensive, and just a bit more time comsuming than any other 30 caliber round.
I use about 10 grains of H110 for 220-240 grain bullets, but they need a 1-8 twist. I have heard that a 1-10 twist will stabilize a 200 grain and lighter bullet. The 7.62x39 case volume is a bit larger than the 300 whisper, so it may take a little bit more powder to make the 150-200 grain bullets move 1000-1050 fps. You will need a chronograph to ensure you have the right velocity if you want to be subsonic without reducing speed too much.
Even if I were to hunt with a suppressed rifle, I would use a full power cartridge like a 223, 7.62x39 or 308 if I planned on shooting past 100 yards. The heavier powder charge and small whip-like sonic boom do not make shooting full power loads in the open uncomfortable. If you can guess your range very accurately out to 200 yards, the subsonic cartridges like the 300 whisper will work for you on deer sized animals. I would not try to hunt bear with subsonic ammo unless I was using a 458 or 510 bullet weighing over 400 grains.
Ranb