I shot a 243AI for a while which although may not have been capable of the MV the WSSM it would not have been too far behind it. I played about with a few bullets in it around the 85grn weight and eventually settled on 85grn HP Sierra Gamekings for a few reasons. They were very, very accurate in my rifle with the twist rate, 1 in 8, I'm not sure what your rifle has, but they also performed very well on deer, good expansion and not excessive meat damage. They are also easily sourced.
I know you are keen to use lighter bullets, but there is a point you reach where the slightly faster MV is not the greatest advantage to be had. If you enjoy shooting at paper and want to do that at distance, then the better BC of the slightly heavier, and longer 85grn+ bullets may offer an advantage that's more usable than the slightly less drop the lighter bullets offer at their greater speeds over shorter distances. But, if you study some ballistic charts of varying bullet weights over different distances, you will see a point where the faster lighter bullets carrying less momentum are overtaken in performance by the heavier bullets. I don't know what distance you intend shooting at but I personally think that in a 6mm chambering the 85grn offers better Allround performance and adaptability than the lighter 70grn.
Just my opinion on it....