Hmm, so just thinking out loud:
Base regeneration rate is 2.25 energy per second, and the change made this value increase by roughly 0.05 per upgrade instead of 0.1.
After 20 HP ups, it should be 2.25+(20*0.05)=3.25 energy/sec.
Pre-nerf, it would have been 2.25+(20*0.1)=4.25 energy/sec.
Over the course of a minute, this adds up to 195 vs 255 - a difference of 60 energy (seeker costs 150 energy, so 1 seeker per minute either way). This means that pre-nerf, you'd get 1 additional seeker per raven every 2.5 minutes.
This change was suggested to me by a couple people when discussing the issue of Ravens acting like ultimate heroes in the lategame - personally, I wanted to nerf the damage scaling instead of the regeneration rate scaling, though this wasn't a popular opinion.
I'll have to look into capping the regen at a certain rate or having it fall off after a certain number of upgrades - not sure if these options are doable with the way it's currently set up.
Other ideas also welcome