You're underestimating the effects of selective assortment (marriage assortment is a prime real-life example of this: the rich marry the rich). Those with +100% eidolons will preferentially friend those with +100% eidolons. Thus the case where he has 2x +100% eidolons and you have 0 is more likely than the case where you have 1x (from friend) and he has 1x (his own). In addition, since MVPs select for outliers, even if he doesn't have a second +100% eidolon to bring, someone else likely will, and that person will end up with the MVP (especially in Ragnaroks).
So from my perspective, the +100% eidolons will be far more disruptive to competitive balance, both in effect per encounter and encounter frequency.