The TSX bullet produces less pressure with the same powder charge simply because there is less bearing surface with the TSX bullet vs the standard X-bullet. Barnes places in each box of TSX bullets, as well as on their website, instructions to use the same data as for the X-bullet, and that you can increase the maximum powder charge up to two full grains. So I approached that like I do any maximum loading, using all the safety procedures until I've reached my accuracy/velocity (chronographed)
objectives, or start observing excess pressure signs (usually measurements just above the case head with a micrometer). Whichever comes first. With the cartridges I use triple shock bullets in, I've increased
the powder charge by .5-gr increments, usually stopping at 1.5-gr additional powder over the max load data for regular X-Bullets.
Until Barnes comes out with a new reloading manual, that is all we have to go with. At the same time, it is reasonable to "assume" that Barnes would not intentionally publish an increase of 2 full grains over their maximum load data if they weren't reasonably assured it was safe. Still,
I would strongly recommend all safety procedures be used anytime working up maximum loads.