This is all depending on several what ifs:
1) the prayers actually take levels literally and not just in effect
2) the mage has not potted up
3) the mage does not counter said prayer with a prayer of their own
1. This does need to be tested out. Does a "drain" show up on your stats as a literal -X amount? If not, this update just got a lot better.
2. Magic potions have a fixed boost of 5, unlike melee potions which boost by a percentage amount and have a greater impact. Even if you have a magic potion with you and assuming 99 as a base, your magic level would still only go from 89 to 94, which is barely enough for ice barrage. Even then, the stat drain continues up to 20% or 25% of your magic level. Either keep sipping pots, or face not being able to use spells.
3. Leech Magic boost starts off at 5% and rises to 10%. Leech Magic drain starts off at 10% and rises to 25%. Which is going to win?