The 500cc scooters are well capable of motorway cruising with quite a bit to spare, and should all top out somewhere around the magic "ton" (some more comfortably so than others). The real biggies like the 650cc Burgman should manage a bit more than that, but to my mind the trade-off is too great. It's nice wafting around the big roads serenely, but a quarter ton scooter is a big lump to weave through London.
Even some of the sub-500cc class should be fine for the motorways. The 400cc's are not far behind the 500's with a good 90mph+ on tap. The 250cc's can generally manage 80mph, but you've not got very much in reserve for overtaking or getting out of trouble.
Your visor fogging problem is normal. It's caused by your warm moist breath coming in to contact with a cold surface and the water condensing out. Having the visor open a tad is helpful but, as you've discovered, it can be a pain in the ass sometimes.
There are many anti-fogging products and even more tricks of the trade to help. One of the simplest is to coat the inside of you visor with a detergent (a couple of drops of washing-up liquid or shampoo applied with a cloth should suffice) which prevents water droplets forming. However, if you ask a roomful of bikers about anti-fogging remedies you will probably find that each one has a different answer and there will be a fair bit of argument and disagreement.
Because I wear glasses which can also suffer from fogging, I use a Respro Foggy mask. It's a neoprene mask that fits inside a full-face helmet using velcro tabs. The idea is that when you put the helmet on, it forms a seal across your cheeks and nose so that, when you breath out, your breath can only escape downwards out of the bottom of the helmet and not upwards towards the visor. It takes a bit of time to get it fitted just right to be comfortable and give a good seal, but it works OK for me.
HTH
one TN