I am really not sure about Atrox being a great breed for a pvp engie. I would take solitus over atrox any time for that. Mongo Rage is cool if you have an impressive alpha you need to get through. The successful pvp engies I know use rifles or energy carbines. AS hits every time, anyway, the bots do much/most of the damage in pvp.
Solitus beta genome heals for around 3500 HP. I doubt an atrox has that much more HP than a solitus, and having lower HP and more healing has some real advantages in PvP - get hit for less but heal for more.
Atrox sure rocks in pvp for other professions. Enfos, MAs, advies. But I do not see their advantage for us. I would go so far as to say that wit of the atrox is of greater use in pvp than mongo rage.
Add to all this that MR has its greatest use against evadig professions, while we are already good at killing those. I have not that much experience with pvp myself, but MAs and shades seemed like the easiest opponents so far. Those and soldiers.
I had my biggest probs with the professions that mess with our pets, like crats. I do not see how MR helps against those.
My 2ct from aperson not so experienced in pvp. ^^