It's a point of diminishing returns. If you recall, a few years ago the factory version of this idea was the .25 WSSM. That cartridge eventually died for a number of reasons, not the least of which was that it had serious problems feeding from magazines.
Your concept could work, but you might have an easier time with an obvious factory offering: the .257 Weatherby. Another choice is the time-tested .25-06. Both of those cartridges require .30-06 length actions, but that is about their only limitation from a hunting perspective. The Weatherby should get that .25 caliber bullet going as fast as you will ever need. Even then, your barrel life will be LESS than the .264 Winchester Magnum, which was a notorious barrel-burner. The .25-06 (or possibly a .25-284) would be about the maximum case capacity you can go with if you want reasonable barrel life.
From a benchrest perspective, shorter actions are theoretically more accurate because they are stiffer. However, I suspect that your goal is not to develop a new cartridge for competition because if you were that kind of advanced competitive shooter, you'd be teaching us about design, rather than asking for our thoughts.
In the end, as great as these new case designs are, there was a great deal of logic behind the "old-fashioned" designs, too. There is a reason why the .270, 7mm Remington Magnum and .300 Winchester Magnum are still kings of the roost today when it comes to practical, workable long-range hunting cartridges. Like I said, there is a point of diminishing returns where the downsides (poor barrel life, feeding problems, "fussiness") tend to outweigh the increases in velocity. In any event, increases in velocity get incrementally smaller as you increase case capacity. Think of a .308. You need to add at least 10 grains more powder to get the extra hundred fps of the .30-06. Then you need to add at least 15 more grains to get the additional 250 fps of the .300 Winchester Magnum. Then you need to add at least 25 more grains to get the extra 250 fps of the .300 Remington Ultra Mag. Each step along the way, you have to go with slower and slower powders, and your choices get smaller and more finicky. The cartridge you described would need to use the slowest powders out there.
If you do go through with this project, please let us know how it all works out because we all learn by watching what each other does.
