Semi-related to Does the target of a grab-based power remain grabbed after the attack?, there are a few different ways in which the Grab-based Flaw can work. One is that you have to make the attack roll for grabbing the person, they make their resistance roll against the grab, and then they save against the DC of the Grab-based power. One is that it works as an option to replace the Strength Damage as a Standard action against someone who you're grabbing. And, honestly, both cases might apply depending on how you interpret the rules text.
If it's the first case, it's got a clear trade-off, much like the Grab itself. The attack bonus to Grab, and the rank of either Strength or the grabbing effect, can't exceed PL*2 just like any attack, so similarly, a power that just adds an additional resistance check would be similarly PL-limited.
If it's the second case, then we're in more ambiguous waters, since the Standard action to inflict damage in a Grab doesn't mention an attack roll, and in prior editions explicitly didn't require a Strength check, basically modeling that someone who has a hold on you just squeezes it tighter. Thus, it could be argued that this should be limited to campaign PL, just like other attacks that lack an attack roll.
So, is a Grab-based power limited by the attack bonus or the Campaign PL?