Having ridden a 650 in the last two year's Super Twins Manx, I can assure you that these bikes seem SO slow round the TT course.
For most of the way it's 5th and 6th gear, and they never seem too quick for the course. 600s are around the horsepower that 750s had when Hizzy complained that they were too fast for the course. Interesting.
If you ask the modern rider, I think they would tend to  agree that a Superbike round the TT course is a real handful, and only a top rider at the peak of his strength can get the best out of them. That's always been the case. They were saying that about Guthrie, Frith, Simpson, Woods, and Daniell before WW2.
I'm sure that a 600 would be a fabulous bike to race round the TT course, I'd love to have a go. But I'm sure that I could not now cope with a Superbike.
It should not escape your notice that the course has really changed over the years, and the mindless obliteration of Windy and Brandish, has done nothing to enhance the quality of the course. It has merely highlighted how fast the top bikes are travelling around most of the course, i.e. between 160 and 190mph.
As Alfie Noakes has said, quite rightly, are there any less accidents in the smaller classes? I really don't think so, but I'm sure that there a statistics to show which way the accident figures are going.