I can understand wanting to use a .458 on a buffalo. I am not sure that I get trying to turn it into a .375 with a 300 gr bullet. On the other hand, a 300 gr .375 will handle PG as far as any PH will be comfortable a client shooting, and it will lethally, and very accurately, drill a buffalo. The Lott is a fairly specialized thumper. And as we have discussed on a previous thread, I can't conceive of a real world advantage in trying to create a radically different light weight load for plains game in that caliber. If I were bringing a Lott on a general game hunt (DG and PG), I'd bring two rifles. If I wanted to do it with a single caliber, my preference, by a large margin, would be a .375 with 300 gr bullets. My second choice would be a .416 with a 400 gr bullet. The only data on such a light bullet I could find was a 325 gr Hornady with a BC of .230. .230! That is about like shooting an ashtray. In comparison, a 270 gr .375 provides a BC of .480. I don't care how big the muzzle is, you still have to penetrate game and carry velocity.
Such a load would likely work as a 150-175 yard deer load, but ...... I guess I am all about proportion when it comes to game animals. For me deer start around 6 mm and end around 7.62.
I suppose it could be a practice load for punching paper, but I am a strong believer in becoming comfortable with a rifle as it was meant to be used. Sure, use cheaper bullets. But, I never download one. You'll develop muscle memory cycling that rifle at one level and are almost guaranteed to be startled in the field when you need to recover from that first shot RIGHT NOW to deal with something angry, hairy, hostile, and inbound..