I agree, it was mostly for specific threats like Tyranitar-Mega:Quite frankly, Deoxys-Speed is a pretty poor mon in the metagame. Especially if it is a suicide lead without Mold Breaker, as you are just asking for Magic Bounce to come in, since your Psycho boost does like no damage. I really don't understand the point of Magic guard when you are leading with it so it won't switch in to hazards and the only mon this helps against is Sand Stream TTar which isn't that common anyway.
Even if you want to use a suicide lead Mega Gengar is usually better, as it can provide offensive presence, as well as bluffing to be the more common offensive setup Mold Breaker set.
Deo-S has 95 base attack so it is not going to be doing much even with 200 BP reversal, if you want something gimmicky like that running Counter/Mirror Coat would be better. On this particular set Spectral Thief might even be a option if he didn't mention that this is a lead.
252 Atk Deoxys-Speed Reversal (200 BP) vs. 252 HP / 252 Def Tyranitar-Mega: 416-492 (102.9 - 121.7%) -- guaranteed OHKO
Counter and Mirror Coat can be risky, especially on a Mixed sweeper that can use either, such as Rayquaza-Mega.
And I know it has low Attack and feel Destiny Bond is best.