Hi Jay,
Only used on roe deer and corsican ram but know some hunters who use it very effectively on wild boar (stalking not drivent hunt) chamois and red deer doe. If your bullet is well placed in boiler room there's no reason not to use on whitetail deer.
I use RWS KS 123grs with great satisfaction and i hand load also with 125 partition. The 120NBT look a very good choice for deer, not far from a 120grs 257AI/25-06. If the 257Roberts is good for whitetail the 6,5x57 will do the same (same case and almost same diameter 257-264)
