The WSM is going to be harder on barrels than most other 6.5's... that is the price one pays for performance. Allen Magnums aren't renowned for long bbl life either, but as Kirby has shown us, proper care & use can lead to a throat life most thought impossible.
My 6.5WSM is wearing a 1-8.4" Obermeyer, the WSM probably won't need a 1-8" to stabilize a 140, I think the 1-8.5-ish should be considered the absolute basement of twist rate. My intuition tells me the 1-8.5 will still Toast 140 VLD's at the velocity potential of the WSM. I would be pretty surprised if a 1-9" wouldn't stabilize a 140 even at or near sea level with a long barrel, however you stated 26" IIRC.
I have 3 or 4 older Shilens, they shoot very well. But, I have also heard that the new ones don't shoot nearly as well as stated above. There are plenty of barrel manufacturers who make good pre-fits, it may be worth your time to contact a few.
All in all, there is nothing wrong with the 6.5WSM, if it fits the scenario. If you are looking for high volume target shooter, you might look elsewhere
If you are looking for top performance for a short action 6.5 Hunting Rifle, you will be hard pressed to find better. If all else fails, spend the extra $150, have the barrel melonited & add the extra life associated with that process.