My wife has a 6.5-284 Shilen (drop in) for her Savage 12 Varminter. Fantastic cartridge, very easy to tune for. HEAVY hitter on coyotes to 700yrds (farthest she's had so far), and great performance on deer. Like Ridge mentioned, if you're only wanting a 400yrd rifle, then the 6.5-284 is probably not going to benefit you much. The 12 is a short action. For really long seats, it's a single loading proposition, but it wasn't a problem getting things tuned to a COL that will run in her repeater.

There's actually been a shift for long range shooters back to the .284 Winchester, and the improved .284 Shehane (approx 100fps improved). The 6.5-284 is a barrel burner. A little nicer on barrels than a 7mm WSM, but not much. Hitting the magic 2950 with a 180grn Berger (published BC = 0.684)from the 284win might exceed your reloading manual, but there are plenty of guys doing it over the bench (Reloader 17). My next barrel will be a .284win Shilen...