I like the idea of buffing up the spell. But i think there's another better solution: I think arcane spellcaster's spells that enables melees should be inferior to cleric ones of the same level (Divine Power [5] > Tenser's Transformation [6]).
... Solution: introduce a level 8 or 9 spell that works like Tenser's Trasformation but with better benefits. This avoids the massive scrolling from UMD classes of a powered up spell.
Example:
Tenser's Hezrou Channeling
Trasmutation 8
Duration: like rage spell (dismissable)
You summon an Hezrou to posses your body, gaining infernal power, greatly improving your melee prowess but you have a slight chance of loose control to the possesing entity. During the spell duration, each minute you have to make a will save against a DC of ?? or be stunned for 3-6 seconds. Your BAB increases to the one of a fighter of your class level, you gain profane bonus to strength (+8) and constitution (+4), a natural armor bonus of (+4), as well as a DR ??/ Good. Casting spells becomes more difficult: you suffer an extra arcane spell failure (let's say 20%) while possesed.
Just an idea.