I have loaded the 180 grain Speer flatpoint in the 35 Rem, 356 Win, and 358 Win and think that it is an excellent whitetail bullet in those rather "mild" 0.358" bore cartridges.
When I first had a 356, I shot 1 whitetail doe with the 220 grain Speer flatpoint and it went through like it was a solid or fmj. I then loaded up some of the 180 grain Speer flatpoints and shot another whitetail doe, this was during our December-January late antlerless season, that resulted in one of the quickest bang-flop kills I've experienced.