spw,
The best way to see if more speed will help is to see the effect on ROD. If the ROD does not go up measurably, than the speed will not be a bad idea, as long as the landing is to a hard surface. Extra touch down speed to a sod surface is a real problem.
If the ROD goes up a bunch, more than 10% than the landing task is getting much more difficult, and the extra speed is probably hurting you.
Remember that for the same IAS on descent, your ground speed is already increasing with altitude (about 2% per thousand feet) so that your landing task at constant speed is more difficult at altitude.
Same argument goes for rpm, exactly. If the rpm increase costs little ROD, then go for it.
Most likely, you will experiment in descent and discover that the flight manual procedures work pretty well as is.
Remember, the gross weight for the helo changes with altitude, follow the charts in the flight manual. The auto should be performed at no more than the weight that you can hover at, IGE, and preferable at a much lower weight for routine training. Those who advocate doing all routine training at the worst case conditions should also try practice bleeding, just in case they have a car accident. Same logic (THAT should keep this thread going!!)
Blender, I think the Jet ranger is the best autorotative helo on the planet, so don't use it as the acid test for altitude effects on helos! Those with low inertia and high ROD will get esty on you fast at altitude. I know of no helo that is the same at altitude as they are at sea level, ALL are worse.