However, my experience with bullets like the TSX, E-Tip and GMX in general is they kill better when really zipping along. Don't see many advantages in them at muzzle velocities much under 3000 fps--except, of course, where non-lead bullets are mandated by law.
I can only compare a 160 TTSX to a 185 TTSX, but faster does make a difference with these. You definitely trade a bit of penetration by going light and fast, and thus opening wider. The internal damage is greater with speed though.
It seems to me there is a market for making them open wider and/or at slower impact velocities. That's just an impression though and not really based on much fact. In fairness the Barnes rep told me the .338 185 while listed as a minimum of 1800, actually went lower than that. I don't know how they define "opening" though. Although look at the Nosler site and compare the E-Tip pictures to the Partition (or Accubond) pictures at ~2000FPS. I have not seen any pictures of the GMX. I would try them if I thought they opened a bit wider at slower velocities than a TTSX. This would make them a better fit for slower cartridges and longer ranges. I may have to try and come up with a test this summer to see how the TTSX, GMX and E-Tip open at ~1800 fps.
Hope more with GMX experience chime in as I would be interested.
