I have trialled the filter slide, & have had no dew problems as yet. I suppose I could put a heating strip around it if need be.
That's my son in the photo (he's 170cm, although he will have grown a centimetre by the time I post this
). I am 185cm, and eyepiece height at zenith is 180cm. One rarely observes at the zenith, so it is possible to use an observing stool most of the time.
As for the question about going even faster: Nagler has a paracorr in development for F/3 - F/3.5 mirrors. But making decent mirrors that fast is very difficult.