While my 6.5cm Barrel on my MWS is a 20 incher, I do have a 13.5 inch 308 barrel that I really like, and I see no reason why the same would not be true of a shorter 6.5. I am actually considering to have it chopped back to 16.5, dimpled and mounted into their new extended length upper. The 13.5 is indeed a little blasty, but since most of my stuff is going to be surprpessed, its all good.
I do most of my shooting in WV and considering the enviroment, we are pretty much limited to 1000. There are places where we may be able to push that range out, but for the most part, we shoot a lot in that 400 to 800 yard window. If I was out west and that window was extended to say past 1k, Longer barrels are clearly the better choice.
Question, Is there a reason you are considering a 6.5cm over a 6.5 grendle? I only ask as for the ranges that you are shooting, The grendle is more than capable of fulfilling your task and doing it almost a pound lighter than a 6.5cm. While I am not a hunter, I think that weight is something that important in a field rifle.