I'll go off the reservation here and opine that you may want to consider handloading for that rifle.
In over 55 years of hunting, I have not fired a factory round in any of my game rifles. If you want both accuracy AND performance, handloading is the only definitive way to find that pet load.
I'm a believer in both Barnes TTSX and Nosler Partitions, both having given me stellar performance on mule deer and big bull elk. Complete penetration of large bulls from any reasonable distance has been my experience.
I'll also go into dangerous territory to ask why you feel you "need" a .300 Mag for whitetails? I'd think a nice light 7mm-08 would be about perfect, and chances are you'll shoot it much better than a .300 Boomer. Add in less muzzle blast, less recoil, more rounds in the magazine, shorter barrel needed, lighter rifle with shorter bolt throw and you begin to see practicality over perceived "need".
Just my opinion, just my experience based on hunting large game for over five decades. Take it for what it's worth.
(P.S. Most of my bull elk have been taken with a M70 Win in .338-06 Ackley...no magnum needed, and these are BIG bulls. And the last raghorn 5x5 bull to come home was taken with my light mountain rifle, a Titanium 7mm-08 Ackley shooting 120gr Barnes TTSX.
'Jus sayin' )