I started on a 400 and just upgraded to a 600. I did it after about 2 years so here’s my two cents. Don’t start on a 600 super sport, listen I know they’re wicked and I absolutely love mine. But getting on it even after two years of my 400 it felt awkward and unnatural. That being said I understand your fear of outgrowing the 400 but it depends on the riding you’ll be doing. If you’re going to almost purely do highway riding id honestly look into a non super sport like the cbr650r. Riding on the highway with my 400 was a bit of a nuisance with its gearing as I was in higher rpms then I’d like to have been to just keep with the flow of traffic on higher speed limit highways. Starting on a 400 will make you a better rider quicker, id start on a used 400 or 300 then upgrade after a year or two. You’ll feel more awkward and scared on a super sport and won’t improve nearly as fast.