I'm not sure I understand what you're saying -- TRG-S IS the Sako M995. But anyways, glad if this information is of use, I know first hand there's not much information on the web regarding the TRG-S. ( the complete owner's manual can be found at Sako Finland - Manufacturer of High Quality Firearms
/ old models, I can't give you the exact link as certain websites are blocked at my work )
Yes, replacing the original stock means getting a new bottom metal, as the original is permanently attached to the stock. This is exactly why I bought the TRG-41 chassis and stock when I happened to come across one second hand.
If you have to go for a custom bottom metal, consider having it done for AIAW magazines instead. TRG magazines cost and arm and a leg. On the other hand, they do allow for longer COAL so do your homework before you commit to it, if kissing the lands is your thing. With AI mags, you won't be able to, AFAIK.
I personally see no reason to touch the barrel contour when rebarreling but then again it's a personal decision. What I *am* wary of, though, is the original stock's structural integrity when facing Wby/LM sized chamberings. But the insides of McMillan stocks aren't exactly my favourite material either -- which is why my 7RM Weatherby is bedded into an A5 with an aluminum block acting as a recoil lug counterpart and front pillar...
Barrel length? 23,26,28,30. All a matter of preference really. I know velocities do not drop as dramatically in .338 as they do in 6.5mm, for example, but sure there is a difference when going for shorter barrels. It all depends on what your intended use is -- for a ELR rifle, go as long as you can and while you're at it, chamber the rifle in 338LM AI. For anything resembling a carry rifle, I'm not so sure a 22-24" is such a bad idea. I could certainly see having one built myself. Admittedly 26-28" is a safe choice, for sure.
Short barreled 338LM TRGs sure do look purty next to more traditional LR rifles, especially when wearing a Gen2 folder rear end.