Hoovers at Williamsport 1000 yard range uses IMR 4350 in their 6.5/284s
Some of the top shooters at Camp Perry shooting 6.5/284s are using the IMR4350 too.
H4831SC is a good one to but, IMR4350 works best in my 6.5/284.
I've though about this,, but won't try it. The burn rate would seem to be too fast, and the 6.5 bore is pretty small to be messing around with pressure like that. If you want an extreme series powder, try H4831 or H4831SC. Same coating as Varget, but burns in the RL 22, H1000 range.
I think John uses 30" Kriegers with the 4350. A charge of like 48 or 49gr is getting all his rifles great MV and long intervals between cleaning.(better than 100 shots at times) He has relativy small chambers and must neck turn, so that may aid in the MV, as well as the extra inches on the barrel, but from our conversation, he and his family are having great success with this powder. I'd try it, but I really like the extreme powder form Hodgdons.
I use 140 gr Sierras with 44-45.5 grains of R-22, with acceptable pressure. Shoots great, except it doesnt meter to well. Also used the 85 gr sierras with H450 and they also shoot great. I would think with lighter bullets Varget might be great. Might try it later, but I only got 2 lbs of varget, while I got 2 kegs each (16 lbs) R-22, H-450. Have to use it up first.