I once bought a new lightweight Winchester Model 70 in .280 with a factory laminated stock free freefloating stock to use as a travel rifle. I sighted in the gun in KY, travelled to Wyoming, sighted in the rifle and it was 1-1/2" high at 100yds. Two days later, hit an antelope dead center at 125. The next day, I shot twice at a mule deer at 100 off a rest and missed so far the deer only looked up. I shot a third time and held even with it's breast and made a heart shot. I suspected I had banged the scope but later examination showed the previously floating barrel was in tight contact with the stock. Tagged out so I cased the gun and came home. A couple of days later I went to the range to re-zero and the first shot was exactly 1-1/2" high. The stock was obviously unstable and changing climates had induced a warp. Even laminated stocks are not immune.
I have built 50-100 custom rifles in the last 40 years including many custom unlimited target rifles, and I no longer trust wooden stocks at all with inletting around the barrel. If you are going to use a wooden stock it should be freefloated to the extreme. That is what I do with target rifles (use a 1-1/4" barrel channel).
For the stock you have, if you can detect warpage and it not even fitted, I would not spend any time on it. Mauser stocks semi-inletted are readily available on eBay and I would simply buy another one. Working on a stock that has a warpage issue is not worth it unless it is an exceptional piece of wood. Even then it is more likely you will always have problems if the grain is fancy.
An upward warp is the worst case for accuracy. The only two things that will help are carbon fiber threads or tape on the outside and bottom which will look terrible or inset a thin vertical aluminum or stainless bar into the bottom of the forearm groove set in epoxy. If this asporter stock it is doubtful that this will be tall enough to give much resistance but it is the right direction.
Steaming the forend will likely cause more movement in the direction of the bend. I don't believe the groove will help any (it is in the wrong plane to take any pressure off the bend. If you want to try, I think you need to just try a steam bend downward. Based on my experience with boatbuilding, I would try a boiling water bath for 30 or 45 minutes with the fore end submerged and then clamping it upside down on a table top with a small block under it at each end of the barrel channel. That will give it a small reverse curve to hopefully spring back to straight after drying and cooling under clamp pressure. Given the flexibility of a barrel I don't think you can bring it straight with clamping it into the barrel. Start with small blocks and retry with larger is you don't end up where you need to be.
