McKie,
I've never used RL15, so I can't really answer that question. From what I've read, it appears that 35 Whelen users generally like it for max velocity and accuracy. Regarding IMR4320, I've only used it with the Speer 220 gr bullet in my 350 rem Mag. In Speer's latest manual, it only lists a bit over 2,600 fps for the max load with this bullet. So, I'm not sure you could get up to almost 2,900 fps with the Barnes 200 TSX, though once again I've never tried. Sorry I couldn't be more helpful. Perhaps someone with much more reloading experience than me will chime in. I'm not what you'd call an adventureous type, when it comes to reloading. I stick only to Reloading Manual loads.