The MP drain should be on Enhanced spellbade GCDs as it would recover MP based on good RDM play as opposed to spamming an ineffective skill to get back MP. Every other job that uses MP has a skill as part of a rotation or a free oGCD to gain their MP back PLD gets it back during their DPS and DoT combo for example. It would likely be a fixed amount, and perhaps only on the final hit (Enhanced Redoublement) since RDM doesn't really have MP issues before Verraise/Vercure. Acceleration could also be used as a small MP gain, given its name, it implies it accelerates the RDM's mana stores. So naturally it would make sense to give it like a 5% MP restore or something (it can be used more often than Aetherflow)