I considered running the 300XTP through mine, but after some sober second thought, I decided it might not be the wisest move to run a bullet designed for impacts <1400 FPS at 2K or higher.
IMO the .429 pistol bullets have no place in the 444 unless loaded way down to handgun velocities. The only exception I might make to that is the 250 Partition or similar. The XTP's and Gold Dot bullets are not constructed for 444 use.
For the OP, what model are you shooting? In the 2 I load for I use 44.4 grains of 4198 and it shoots beautifully. These are both older microgroove 444 rifles. A friend is shooting the XLR version and has been more than happy using the LeverEvolution factory rounds - so much so it is the one rifle he owns that he has not asked me to reload for.