As for the gun - the M7 is great. The only thing that I don't like about it is that, in standard calibers, it only has a 20" barrel, whereas I like at least 22".
As for the cartridge, it is about as powerful as the 7mm Rem Mag - which is pretty darn powerful. While the WSMs have become quite popular, the SAUMs really have not. On the other hand, it appears to me that while the WSMs are popular in the .300 and .270, the SAUM seems more popular in the SAUM.
A good thing about the 7mm SAUM is that Remington offers premium bullets for it - like the Nosler partion.
Now, I think that the M7 in the SAUM has a 22" barrel, which is longer than 20", but shorter than than 24" or longer for magnum loads. Still, the M7 in 7mm SAUM sounds like a great combo. Just keep in mind that a powerful load in such a small little, light gun might make for some pretty tough recoil.
Zachary