RE: What is the appropriate APR for HBD savings according to you ?

Yes, this point is definitely worth mentioning - HBD interest means rewarding inactivity. A part of the Reward Pool is distributed among those, who simply deposit their money in Savings. I believe distributing the RP among creators, curators, witnesses and people helping Hive grow through projects supported by community Proposals would be more beneficient to the network. I don't say it is definitely true, I may be missing some important bit, yet this is what the common sense suggests :)