It's not a bad system Becq, but how does that work with immunity (which has only one level that costs 8 refresh) and the stacked catch?
Ah, PI -- I hadn't really had that in mind when writing my previous post, and I agree with Sanctaphrax that is needs to be handled a bit differently.
But as a first cut, here are some thoughts:
First, Physical Immunity should probably be separated (at least conceptually) into two powers. Full Physical Immunity is the default setting of the power, and represents immunity to everything that isn't The Catch. It's basically the next level of Toughness beyond Mythic; as such, it should count as four levels of Toughness/Recovery powers for purposes of the rebate (one level per two refresh).
Specific Physical Immunity is immunity only to a specific subset of attack types, and is what you get when you add a Stacked Catch to Physical Immunity. The normal Catch doesn't apply, so it counts as zero levels for purposes of the Catch rebate, but should have a cost reduction based on how commonly it offers protection from attacks.
Perhaps Specific Physical Immunity should be priced something like this:
Common PI: A significant number of opponents will be at a disadvantage against the character. Cost: -4
Uncommon PI: An opponent or two per fight or a hostile group or two per session will be at a disadvantage against the character, or a larger number will be inconvenienced. Cost -2.
Rare PI: Perhaps as few as several opponents per campaign will be at a disadvantage, or a larger number will be inconvenienced. Cost -1.
For purposes of the above, assume an opponent is 'disadvantaged' if they are unable to hurt the character at all (because they either don't have an alternate attack or are unlikely to realize that an alternate attack would improve things), and that an opponent is 'inconvenienced' if they are able to bypass the PI, and can quickly recognize the need to, but with significantly inferior attacks. Also note that if the number of opponents that are disadvantaged even begins to approach a majority, then the character should probably be buying Full Physical Immunity (possibly with a serious Catch) rather than Specific Physical Immunity.
This may well need significant adjustment.