Sorry, after playing with these a couple hours.
Not really hotter than the Abraxas, a bit more aggressive in the highs, and where the Abraxas is breathing and purring really openly, the True Grits are fattier and juicier. One could say they are more modern indeed. They have this chunkiness in the low-mids but the lowest of the lows are actually quite tight. I don't love them more than the Abraxas (which truly are a gem of a pickup), but they are going to stay in for a while until I find a guitar that will be their final home. Probably going to stay on a full mahogany guitar, been eyeing getting a Gibson V for a while.
Anyway, due to their more compressed character, though slightly weaker output, I would definitively say they sound more modern than the Rebel Yell. They have more tighter lows, more low-mids and more aggressive trebles when the RY has this small but round low end, big open hairy push in the mids and high-mids, and more vintage trebles.
Hopefully this small review will come fatten Google results and help with the lack of True Grit and Bootcamp reviews around here.