It is basically the only cartridge class that Weatherby can't claim top dog status in, "commercially available", and the motto around here is "Nothing shoots flatter, etc", added with the overwhelming popularity of long range shooting now days, there probably is a point and a place for the 7mm-300, even in a crowded market.
The 7mm-378 may be a bit much though, unless maybe they shortend it some, although you'd probably just be getting back down around 300 case capacity so that would probably be foolish, but perhaps the shorter, fatter burn column would make it less finicky and easier to work loads up?
It would definitely hurt the original 7 Bee's life span, which is an already great cartridge, but I wouldn't see it effecting the 6.5-300's much.
All in all as a huge fan of Weatherby, I would most certainly be interested in a super fast, new 7mm, although personally I would much rather see them hit a market in which they have nothing offered, and introduce a true varmint round, 20-224, 220 Rocket, or heck, even another run of the 224!