Do Summer starters have a lower median LSAT/GPA?
I could be wrong, but it's my understanding that most (though not all) summer starters are ED applicants. (I also believe that all ED admits end up as summer starters, correct?.)
Given that ED applicants are probably (though not necessarily) more borderline applicants, I would assume that they do, on average, have a slightly lower average numerical profile. (The fact a prof would try to dispute this is completely foreseeable, but also completely meaningless without supporting data.) It's not going to be a huge difference either way.
I'll also second what others have said about summer start -- most people I've spoken to who've done this appear to love it. To the extent they form a clique, it's because they actually get to know each other due to the smaller class size.
Any change that is being discussed is likely the change that no longer allows schools to hide students from ABA reporting students in non fall start programs. It used to be the case that a school would not have to report the statistics of students in a program like the summer program, and so it became a place for those whose numbers were less than impressive (this may even explain why ED was summer start only); now all statistics are reported, so the benefit to hiding people in the summer class no longer exists.