I think this kind of change would make Soraka more interesting and powerful, but I think it could easily be abused. It's a really neat idea, but in practice I think it would simply be very frustrating to play against and, more importantly, take Soraka back into the territory of being a "life and mana battery" instead of someone with combat casting presence.
Allowing her to heal significantly more than she currently does (regardless of her Q strength, which I think is in a good place as a support) will make her optimal play shift more towards building tremendous amounts of hp and mana regen. Currently, a Soraka support has decent heal and infuse numbers, but used on an ally they are low enough that she must always do the dance of "do I sit back and heal - or do I poke and fight?".
Playing Soraka, you're always forced to make awkward choices if you want to play passively - wasting the armor buff on W, asking yourself if you should try to poke with Q so you can reduce your W cooldown, choosing to use E on an ally or enemy - and that's good. S2 super-passive Soraka was undoubtedly really strong, but also bad for gameplay as a whole (and honestly just made her unsatisfying to play).
So creating the choice between a "combat Soraka who trades off stats for other stats" and "passive Soraka who sits back and pumps allies full of power juice" is definitely a good direction, which is why I like your idea :). My argument is just that when players are faced with this choice, regardless of power, I think that 90% of the time higher level players are going to go with the passive option simply because of how strong it is to give HP to allies. Lost a trade and at 200 hp while Soraka was at fountain? She'll show up and donate a bunch of it back to you so you're good to go.
Suggestion: Maybe add on a requirement to this "infusion" heal where she must be in combat? Engaged in combat in the last X seconds, have at least one Starcall stack, be in proximity of an enemy champion? I know these aren't the best solutions, but adding some kind of combat requirement for this channneled heal will help prevent it from being abused.
TL;DR Soraka's heal/infuse are low enough that she is forced to be a combat mage in lane, buffing her ability to heal will make spamming regen and playing super passively a "better" way to play her (which makes her boring). Maybe add some kind of combat requirement? Good idea though!