Steve, I use the 139gr Scenars but point them up to increase their BC and have put a number of 3 shot groups thru the same hole at 100m. If your not pointing then follow Alan's advice and use the the 140-142gr projectiles.
The team USA shot the 6.5x284 in last years world F-Open championships and did very well. The 7mm calibres with 180gr projectiles did most of the winning.
The Lapua brass 6.5x284 is what most of us use for our 284W or 284 Shehane cases, we all expand them up from new brass. Alan is also correct about the need to anneal used brass before expanding up to 284W. I anneal my brass cases after every 5 firings with a bernzamatic propane torch from Bunnings.
There is only a very small ballistic advantage using a 300WM with 210gr projectiles and it involves launching them at higher velocities than the 284W with the 180gr VLDs. The 300WM uses more powder, needs heavier projectiles launched at higher velocities, has increased barrel wear and has a good deal more recoil to contend with in competitions. I dont think it will offer any advantage to you and would be a bear to shoot in a little Tikka.
If you want to shoot 30cal in F-Open then the 300WSM is the better choice. It can launch the 210gr VLDs at 2900fps (300WM good for 3000+fps) from a more efficient case with less recoil. My advice would still be to get the Savage in 6.5x284 if you are serious about F-Open and upgrade to a 7mm barrel later.
It would even be possible to use the Savage as a switch barrel and use the 6.5x284 for the ranges out to 600m and screw on the 7mm barrel and use the 284W for long range.
There is a 3 part article on safclass.com.au that may be of interest to you, all about the pursuit of the elusive ballistic advantage. It also has useful info on getting on target at long range.
Look Here:
http://www.safclass.com.au/BallisticBasics.html
IanP