I would think a roll against 12-Magnitude +(modifiers) + casters spell mastery score. Might be more appropriate.
So, my character can get Lion Claws, and it's a Major Flaw? SOLD!
Um, on second thought... no way, Jose.
5th ed tends to shy away from such "combo deals", where a flaw and an obvious virtue are combined. To disregard the advantages of the virtue, and given the potentially minor drawbacks of the flaw (wear a hat?), I'd veto this. Or, make the first half "unable to disguise", and lose the second half. Purely cosmetic, or a tiny bonus at best.
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Well I could see it as a minor story flaw. Many of those give some kind of benefit but at the price of passing up the ability to have a major story flaw. Then again I'm not sure how much story generating ability this has. Maybe a minor supernatural flaw?
You know, now that I stop to think about it maybe this needs to be a new flaw type:
Warping Flaw: A flaw that can only be gained through magical warping.
Take the above, make it a minor flaw, and stick a "bestial" personality trait of +1 with it and that'd be about balanced.
Re: Inhuman appearance -- see "Repellent" (Major, General, Tainted), RoP:I p. 95, that's pretty much the same thing. Otherwise, as a minor general flaw, there's strong precedent for a simple "Disfigured (animal feature)" (Semita Errabunda's "Amelle" and the Bjornaer Therianthropy initiation script flaw.
I think there may be good value in seeking to find out what variations can be made on existing virtues and flaws -- or at least indicate which ones you are using for reference. 
Incidentally, "Combat Improvisationalist" is unnecessary, as the Brawl skill already covers improvised weapons under the "Bludgeon" heading.
Brawl, with a score in an ability (Martial Art) added to it?
Oh, surely that should be a Major Flaw too! (was for Kwai Chang...)
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