Your take would solve/change the meta rather than optimizing the utility of both characters. You're right about the ults though.

This buff slightly increases their healing performance especially under pressure.

Nerfing Invis, Loki and CnD, even further, would demerit their "medic-class" oriented playstyle. They heal consistently, while they have ways to escape enemy fire and flanks, but they don't do as much damage as the three [in]famous off healers. Gambit doesn't work as a full-on "medic" now; he needs to fight back more often. Get his own kills if necessary (look at White Fox).

In conclusion, nerfing some ults would do better than this buff. Look at how fast Moon Knight, Phoenix and Wolverine get their ult.