After looking up the current thought on how magic accuracy and magic evasion are used to resist spells, I'd say that that change might be too much. Not in and of itself, as it's quite clear that a Dunna GEO can effectively give +190 magic evasion. The problem is that nothing exists in a vacuum. +362 effective magic evasion, on top of the magic evasion on RUN-usable armor (which I want to say tends to be high-ish), that could well make the RUN effectively invulnerable. Especially when you throw bar spells on top of that, although that is for single elements.

From what I have been able to find, Foil does not add magic evasion, but a bonus to regular evasion against TP moves; I have not tested this myself, but I consider the source reliable. It is not enough to simply increase magic evasion (although that goes a long way to helping the problem). Perhaps Foil, in addition to the evasion against TP moves, could give the RUN increased Magic Defense Bonus. RUN already gets Tier 7 Magic Defense Bonus, which is +22 MDB. Have Foil, on top of its current effects, give +Magic Defense Bonus, capping at... I dunno. +22 at 500 Enhancing. If every ~22 points of Enhancing gave +1 MDB, a RUN without any Enhancing Gear or Merits, but capped Enhancing, would get +17 MDB. MDB, as far as I know, has nothing to do with the effects of debuffs, but it could go a long way to make up for the advantage PLD has in general magic damage reduction over RUN.

But then again, perhaps a large increase to just magic evasion is the way to go. In my experiences so far on RUN, the problem has seemed to be not the level of resistance and damage reduction I can achieve, but the limitation to 3 or less elements at once. Which, of course, a flat increase would help resists.