I just got my 2012 Hornady LE & Military catalog and they've included the 55gr GMX loading in their catalog. It looks the same as the superformance GMX loading.
Here are the specifics:
Product # = 81255 (5.56mm pressure)
55gr GMX (no polymer tip that's typical with other GMX bullets. It resembles a non-boattail TSX but with a larger internal HP cavity)
B.C. = .245
S.D. = .157
Advertised Velocity out of a 16" test barrel):
Mz = 3130, 100yd = 2739, 200yd = 2380, 300yd = 2049
16" 1:9 weapon
Measured Velocity = 3193
21" Penetration (pretty amazing for a fully-expanded 55gr .22 cal bullet)
Max Cav = 4" (not nearly as dramatic as the 75gr TAP)
Depth to Max Cav = 5"
Entry = 1"
Retained weight = 55gr
The bullet looked like a perfect mushroom. I suspect that the guilding metal construction will retain its petals through intermediate barriers better than a pure copper design . The profile of the mushroom looks more similar to a conventional bullet versus the TSX that has substantial gap between the petals. I bet this would make a good hunting round as well.
By the way, the gel shot with the 75gr 5.56mm TAp 8126N still looked the most impressive of any .223/5.56mm round. It obviously remains a top pick for unobstructed shots.