If all goes well, the little bullet will do the job - but - some of those big Wyoming mulies are mighty tough and even walk off after hits from bigger cartridges.
I've successfully hunted mule deer with the 6mm Rem and my .25-06, but I think even those bullets are a little light at longer ranges. Just a thought. For longer ranges when the velocity has fallen off, I prefer a bigger bullet, say something in 7mm or .30 caliber to be sure of enough penetration on a 250 pound heavily muscled buck out there at 300+ yards.
BTW - the Berger VLD design works just like gunwriter John Barsness claims - nice penetration into the chest cavity, followed by utter destruction of the lungs & heart. Put that VLD in the boiler room and you're looking at success. I've only taken one mule deer with that bullet, but it worked well last season, and I may well use the VLD again this year.
Regards, Guy