I've always wondered whether the turbine at idle contributes any significant power to uping rrpm
If the needles are joined, there will be power being transmitted. It's how I nearly got caught doing a real one early on. As a result, nowadays I never allow anyone to continue on looking at their intended landing spot in an auto descent if they are doing a power recovery and have joined the needles, as the projected landing spot will be further out than for when there is silence.
You didn't mention whether you are talking about a hover or an inflight scenario. I've mentioned in the past that I thought a lot of turbine pilots get over confident about the ability to do a hover autorotation. A jet engine engineer got in contact with me and said that there are two factors: the fuel control is designed to slowly decrease the fuel flow so that you don't flame the engine out, so in fact when you quickly roll off throttle in the hover, the engine is still developing significant power for most of the hovering autorotation. However, he said that the inertia in the power turbine is substantial and that does help out. So, real life will not be as forgiving because you'll lose the help of the engine.
In a glide, if the needles are split, I don't believe you are getting any help from the engine. Split is split, as far as I know.