Brad, I agree that the 8400 is a bit different beast. I've only handled 84M's and the 84L's... not hunted them.
The reason I personally would "strap an anvil" onto my 8400 is pure and simple: to increase its capability. The 7 WSM chambering has serious legs to it. Obviously a lot factors into how well a rifle + shooter combo can utilize those legs.... the mechanical accuracy has to be there, and the shooter has to have their [bleep] wired pretty tight to really utilize that reach in a light platform.
Since this stuff costs MONEY and my college daughters and house build at 95% complete (oy, those last 5%..... grin) are bleeding me PALE...... I am personally trying to be a nazi about "keeping it real".... what I mean by that is, I will gladly stick a $1000 "anvil" on that rifle IF I can show myself that as a package, rifle + shooter, we "deserve" it. If not, then any that I mentioned- 2.5-8 Leup, 3-10 Swaro, 3-9 Conquest, etc, all give solid 500-yard reach.
Larry commented negatively on the Leup 2.5-8. I've owned 6-8 of them and have had, and still do have, several that track and RTZ just fine within the above parameters- as a solid 500 yard rifle that will hit vitals. As many here posting know, things get exponentially more squirrely as you up the range and precision begins to rule the day. In an optic, that kind of precision resides in Nightforce, SWFA, the high end Bushnell's, etc. All are considerably heavier than lightweight hunting scopes. All have increased utility. The nut of it is- can the rifle/shooter combo UTILIZE that added precision in such a light rifle? You want to have your mind blown, trot up to the optics forum and look in the Litz/tracking thread at the tall target tests EHG put up pics of, shot with various "heavy" scopes on a .308 Montana. There ARE people for whom a Montana is not just a 500 yard rifle. Don't know about the OP's skills, or intentions, in this regard.
