Beating the healer is one thing, the healer having to weaken themselves by losing a big chunk of life is something else.
Again, going back to GW1 infuse was one skill of 8 on your bar. You didn't have to take it. You also had 7 slots to fill with things for healing yourself back up or defend yourself with. Ideally, your monk would be in the back line to use this, not in the middle of things like you want an active Soraka to be.
I have to be pretty absolutionist for this idea because frankly these are 2 different games. An 8v8 furball is a little different than 5v5 split up over the map.
Soraka has no escape, no CC, and without her W (on which would be CD) no defense. It's not like she'd have 7 other skills to fall back on. I'm not even sure how you'd implement it to keep the people who like taking her into a solo lane happy. I'm one of those people who like taking her solo.
And let's be honest, in a team fight where you've got 5 enemy champs with pokes, and CCs, and engages, and nidalee spears, and flashes, you can't have a champ that loses a massive chunk of health, and healing power by extention. She's only have one heal.
TBH if you did this, Nidalee (and maybe a few other champs) would instantly be a hard counter to Soraka which goes directly against riot's prinicples. I started a CB thread (or was it GD) awhile ago on how you can't just do that. So if you nerf soraka's healing to be a lose X amount of life, you have to nerf nidalee's spear the same patch. I know a lot of people want nidalee's spear to be changed, but still...
That's a huge amount of rage you are setting yourself up for in just one patch.
Here's the most important reason not to do this. Soraka is a sustained submission fighter. She wins battles by trying to draw them out as long as possible where she gets the advantage (stacking MR shred). A change like this would completely destroy how she fights.
It basically would put her into a fight with only 3 usable skills (Q,E,R) while other champs have 4. The more she'd use her W with this suggestion, the more of a liability she becomes. This is completely against how she plays and how she needs to play in order to win a fight.
Pros might be able to use this effectively, but sub gold she'd be either A. Trash or B. uber passive which we don't want her to be.
Qing is hard enough bot because some ADCs hate it when you push the lane. Or take their minions. It's also her best offensive skill for bot, and I love it as is and don't want starcall, infuse, or wish changed. And now you want her healing to be a liability as well? Even if it was only 10% that's going to lead to a lot more deaths or back than before.
That's not a whole lot of room for Soraka players to actually play the game. Over in GD fearless is asking Sona players how to make her more rewarding. If you do this change, 6 months from now either him, Vesh, or Rickless is going to be coming into the forums and asking the same thing about soraka.
While I'm thinking about it, you do realize that part of Riot's problem with healing is you aren't give players any tools to interact with them right? What options do other players have in LOL? We don't have domination mesmers, that's for sure. We don't have any champ who can sap X of your mana/energy and do damage to you based on how much you lost. We certainly don't have a lot of viable options for keeping grievous wounds to stick to a champ.
Part of me thinks you guys are trying to fix Soraka the wrong way. When you should be adding levels for more interaction, you are simply pulling the one you have and wonder why people are angry the only real healer is getting nerfed again.