I should also point out that, if the assumption you start with is "the drag on the economy (relative to potential) from credit card fees is equal to the amount of those fees", you should quickly notice that the directionality is wrong.
In my example example, cutting fees to 1.5% (from any level at all, interestingly) requires the economy to expand by 1.5%. That's not a coincidence -- cutting fees to 0.1% would require the economy to expand by 0.1%, except that that's completely implausible; cutting fees further should cause the economy to expand more, not less.
With that in mind, it might make sense to measure against the hypothetical where credit card companies offer their services for free, but even then there is no obvious relationship to the current level of their fees. I have to stand by my assessment that saying Visa/MC are skimming 2.5% of the economy doesn't make sense. How'd we get that number?