I think it would be fair to make the health on kill per 20 seconds scale based upon level. Maybe something like 15/20/25/30/etc., as you level up at 1/2/3/4/etc.
Plz Buff Feast. Not 
Feast. That Feast is fine. Plz Buff the other Feast. The Mastery. Plz.
Feast is meant to be a bit of extra lane sustain, not an alternative to buying Lifesteal. If it was, then it would be too strong to be picked as a mastery.
It's actually extremely strong early on and with 
outheals what a lot of poke supports are able to dish out early on. It's not a final keystone mastery either, so I think it is more than strong enough.
Try comparing it to the equivalent in Cunning or Resolve, Secret Stash or Tough Skin respectively. If you last-hit a single wave, then you have already gained as much health from Feast as you would from the difference between 3x
and 3x
. If you take just a single CS, then you have healed as much as Tough Skin would protect you in total from 10 entire basic attacks.
If anything, Feast should be nerfed to bring it in line with Cunning and Resolve. Restoring like 6 health per CS would actually be pretty generous when compared to the alternative masteries.
At best it's 1 hp per second
