actualy the rearward thrust on the WSM and WSSM's is less than alot of other rounds like the 22-250 and 220 Swift and others that the case has alot of taper and their isin't any trouble with them running into frame battering problems when they are loaded to sane pressures.
The Encores barrel is only 1" in diameter at the case head which on the WSSM , WSM and RUM's is well over .500" leaving less than .250" of chamber wall and thats unsafe anyway you look at it. I have seen Encores that were screwed up for guys trying to push a 30-06 to 300 mag velocities , the barrle had bulged to the point that it woulden't close anymore. Now that is a 30-06 case that is alot smaller in diameter than the WSSM and has more bolt thrust and it screwed up the barrel from pressure.
In order to get the WSSM to shoot well safely your going to be running it at lower pressures than it was desgined for which is going to reduce your velocity. I'd be willing to bet that you could runn the regular 243 up to higher velocities and it would be alot more safe , and I know that you can do it with the 243 Ai cause I've seen it done several time with bolt guns.
I'm not sure what it is that you looking for out of the 243WSSM case but I personaly think that their are alot better choices out their that will give you better performance with less headache.