Here's how you Spriest:
Priestz @ Ursin - Community - World of Warcraft
Spells no longer have any base damage, which means that 100% of your spells damage comes from spell power. Devouring Plague does 300% of your spell power in damage + another 100% over 6 seconds. On players with 4-5k HP you would have to have to have 1334-1667 spell power for DP to one shot. Factor in Mind Blast damage (180% of spell power), and you'd have to have between 833 and 1041 spell power for Mind Blast + Devouring Plague to one shot someone with that glyph.
Since the average caster/healer is sitting around 220 spell power, I would say that the Mind Blast glyph is terrible @ 29, considering it only gives you 3 orbs on the first time you hit your target with MB AND adds a 6 second CD to MB. Kinda of a waste if you don't kill your target in the opener. Probably worth if you go glass cannon build and farm F2Ps in skirms all day.
On a good day, with 225 spell power, you can expect to crit for about 1265 on DP and 758 on MB. If the stars align, you can get them both to crit and do about 2k dmg to someone.