I hit 50 once -- coasting -- coming down off Mt. Weather. There are no real straightaways on the descent. There are signs warning you of the tight turns on the way down -- i.e., there are speed-limit advisory signs that you should pay reasonable attention to, until you get a good "feel" for where you need to be super carefull. I agree with flythebike's reccomendation that you not push it much above 40.

US-50 (i.e, Ashby Gap, the continuation of the Mt. Weather descent) can be a semi-busy road; I don't recall what the state of the shoulder is (or was...). If it's clear, traffic-wise, you can just let it go descending through Ashby Gap. It's just a gentle curve to the right to get into Paris; if you're going too fast for the Paris turn-off, you'll hit US-17 shortly anyway and you can just pick up the ride from there.