That is an absolute.
When it comes to headspeed, full size manned helicopters run ~700 ft/sec blade tip speed almost exclusively. Except for the old Huey's used to be over 800 and they had really noisy rotors that bark pretty sharp. That would be around 2,300 rpm for your 800 stretch.
Have you figured out what the disc loading is of your 800 stretch, Tim? Full sized heli's run disc loading of anywhere from about 2.5 up to 15 lbs/sq ft and is one of the reasons they are so much more stable than RC heli's. To get the disc loading up to what say a Robinson R22 has, your 800 stretch would have to have a takeoff weight of 65lbs. And the R22 is just a light utility helicopter.
That should give you idea of the problems you're dealing with a RC heli.
This is why the RC heli's can run so much lower blade tip speed, though, but it's not necessarily a good thing for stability of the machine. I'm not a firm believer in running low headspeeds where you have to use more than about 5.5 degrees of collective pitch to hover it. I have just never seen good results from it. And that's pretty much the "rule of thumb" I have used is to set the collective to 5.5 degrees, and if it don't hover, speed it up until it does. If you have to use excessive collective to hover it, then you lose cyclic response and it becomes unstable.
Adjusting the headspeed is the absolute best way to compensate for carrying a payload with your RC helicopter. I can lift 22lbs with mine, but it won't do it at 1,700 rpm. I run it at 2,150 and it can still maneuver with the load. At 1,700 it can't. So when you put your payload on it, speed it up and add more fuel weight. The increased disc loading, and additional headspeed will keep it stable. It's just an absolute.
When it comes to power consumption, the power increase required to turn the main rotor is exponential as rotor speed increases, not linear. So bumping it up to get more stable characteristic, within reason, really doesn't affect power consumption that much in forward flight - only in hover. I ran several timed flight tests with my 600 electric running 1,800 (where it tail bobs a little bit), vs 1,900 where it's stable. That helicopter has a takeoff weight of 10.05 lbs and amp draw averaging about 17-18 on 6S @ 25 mph. Four timed auto flights and could not measure the difference in flight time. In hover the difference is 22-23 amps vs 23-24. So it hovers on about 500W @ 1,800 vs 540W @ 1,900.
I sometimes think that using a H_RSC_MODE 3 V-throttle curve will yield a more efficient and stable under all conditions electric heli than using MODE 2 with a governor. I use MODE 3 with my little 500 because it has no governor in the ESC. I have not experiment with that with the bigger ones yet. But the V-curve is based on collective pitch so if it hovers at 5.5 degrees with no payload, and requires 6 degrees with a decent load, it should speed the head up some automatically. It's one of the experiments I got on my list of things to do.