http://green.autoblog.com/2011/02/17...-goes-lithium/

Of particular note:

 

I dont know how deep Honda cycles/ discharges their battery pack ( don't own a Honda HDS tool), I'd wager it's conservative. IF I obtain a HDS, I would LOVE to set the discharge cycles from full to 10% SOC ( NiMH ) and increase the amperage discharge rate somewhat....I'd bet it COULD achieve 100 MPG. I am basing this off of rumors that MOST MFG's only cycle batteries 20-30% SOC.

 

I doubt increasing the SoC range would help mileage that much.

From the 1st PDF at http://priuschat.com/forums/gen-ii-p...provement.html (describing the 5 "stages" of Prius operation)
Seriously, if you think about it, in a non-plugin hybrid, all the energy ultimately comes from the ICE. To recapture energy by regen, the ICE had to do work to accelerate you. When you regen, you're incurring losses in the motor/generator, wiring, batteries, and so on. To repurpose that energy, you're incurring losses again in those same areas.

Prius hypermilers aim to minimize amount they add to or remove from the HV pack.

As for SoC range, I can confirm your statement on the 2nd gen Prius. http://privatenrg.com/#Full_SOC has a mapping of the display to SoC values. I also monitor the actual % SoC via ScanGauge. The Prius starts doing funny things around ~75% SoC if you're going down a hill to prevent overcharging (it essentially puts itself in B mode, spins up the ICE and runs it at high RPMs, etc.) It rarely goes below 50% SoC (or 1-2 purple bars) unless you're idling for a long time or creeping a long for a long time, esp. w/AC running.

I don't think I've ever seen SoC % get above 81 or 82% and the only way to get there is by going down a long downhill grade (or maybe force charging by holding down both pedals, which I don't do).