I remember one person said they got better mpg in HV Charge mode, the theory being as you said that it causes the engine to operate in the more ideal part of the power band. But others say they didn't see much difference. Obermd says he tried it and lost about 20% total HV range compared to regular HV mode. But measuring all of this accurately is pretty difficult without repeated, controlled tests using the odometer and tracking actual gas usage.Engineers work with marketing to make decisions. Sometimes, they're not "allowed" to do things the most efficient way possible.
Case in point: In my 2011 Volt, the engine runs in "Normal" mode, unless you change it to another, such as "Mountain" mode. In Mountain mode, the engine revs at a much higher rate, in order to put extra energy back into the battery, in anticipation of needing the extra battery buffer during ascent of...well...mountains. In Mountain mode, the engine noise can be quite noticeable. Some might even call it jarringly loud.
What I found was that if I played with the modes on flat ground, that the vehicle got about 10% better gas mileage if I kept it in Mountain mode while the engine was running. It required significant diligence on my part, to keep track of battery SOC, and turn on Mountain mode when the battery got down to where the GOM reported about 3 miles worth of charge. I'd then leave it in Mountain mode for about 20 minutes, while the vehicle engine (relatively loudly) added a buffer to the battery. I'd then turn it back to regular mode and run off the battery (in EV mode), until it got down to 3 mies EV range again. Repeat.
What this seems to show is that GM made a marketing decision, that drivers would prefer a quieter environment in the Volt, rather then maximizing the efficiency of the system by running the engine at a louder and higher RPM.
I suspect that the nigher RPM put the little engine more into the peak area of its power band, which can be more efficient. It's possible that the same can be said for the Clarity PHEV, but I haven't played with it to the same extent that I did the Volt.
I think one complication with the Clarity compared to the Volt is that in HV Charge mode the engine doesn't run all the time, it seems in many ways like it's just operating in regular HV mode but with a higher set point, so it will still shut off the gas engine at times.