I think you should read what Long Time Long Ranger has to say in this thread before you make the expensive switch.

One quote: "I tested these two at length on my range when the 338 RUM came out in 2001. I quit doing the 338-300 RUM I had been doing for a few years at that time because there was no improvement in performance and the 338 RUM was over the counter."

http://www.longrangehunting.com/forums/f19/338-rum-vs-338-edge-68625/