Last I heard Chey Tac had contracted Sierra to make a 350 and 375 gr version of the 375 cal SMK. As far as I know, the entire lot of bullets will be shipped to Chey Tac industries and not available to the public. I also heard that they would probably be available from CT if you wanted to buy the bullets from them.
In talking with Barney Lawton, the 375 Chey Tac is a hell of a round when using the Lost River Ballistic bullet. It is outperforming the 338 rounds off the 408 case by a bit but these bullets are very spendy and you need the right barrel to get them to shoot well. Barney makes specially designed barrels for these bullets and they also work great for other conventional bullets as well.
Wildcat Bullets is working on getting set up for the 375 cal bullets as well. If that happens I hope to talk Richard into going up in bullet weight. Personally, for a conventional lead core bullet design, 375 gr is to light for me unless it is a tipped bullet. I would much rather see a 400 gr bullet and I hope to talk Richard into making me some for my 375 Allen Magnum when its ready to roll. This is basically the same as the 375 CT but with the shoulder moved farther forward for about 10 grains more capacity just like my 338 AM compared to the 338 CT.
Right now the limiting factor is available bullets. Get a Lawton barrel and you can use the LRB bullets if you can afford to shoot them. With a rifle in this price range bullet price really is not a huge factor if you can afford the rifle but still its a consideration.
Other then that, until Sierra makes available the heavier SMK bullets there are really no good options. The 300 gr Sierra BTSP would be a screamer of a bullet but ballistically not even in the same ball park as any of the upper weight 338 long range bullets.
About the 50 cal based on the 408, that is an impressive round. Make it in a rifle heavier then 15 lbs though, mine is currently waiting for a muzzle brake
upgrade if I ever get around to machining the new brake. 9000 ft/lbs of energy is not a problem and as mentioned, its almost impossible to heat the barrl up!!! I have put five rounds though mine as fast as I could physically handle, around 2 minutes spread and the barrel was barely warm to the touch. But again, recoil with the conventional brake, exciting to say the least!!!