From what I've seen, people won't expect it to be a metal burst aggron, at least not until you beat them with it a few times. Also, with Sableye, you can just pound it with a hard enough move and OHKO it, while you need Mold Breaker or Teravolt with a good enough move to OHKO Aggron, a feat that is very rare even for Aggron that have minimized defenses.
This might just be me, but I go for Will-o-Wisp or a similar move any time I see anything Sturdy, since Metal Burst, Counter, and Mirror Coat are extremely predictable. Mega Sableye's Magic Bounce makes this impossible.
Also, you overestimated the importance of being able to OHKO Mega Aggron. Its HP is low enough that a 2HKO (e.g. an Earth Power from the incredibly common Kyu-B) will not do enough that the attacker is killed by Metal Burst. Sableye also has low HP, so this is more of an issue with Metal Burst as a move than anything else.
One more thing: you can't just hit Mega Sableye with a strong enough move to OHKO. It lives so many attacks and has only one weakness, and the EVs raimon is using allow it to live the most common attack of that type (Mega Diancie's Moonblast).