i think you're right on, for the most part. there are firms focused on t3, t6, t10, and also t14. this is also the case for firms that regularly draw from tier 2, who certainly divide that tier in to different groups.
however, i don't believe it's true that, within tier 1, schools ranked 15-50 are all treated the same. for example, there's a big difference in firms recruiting from something like texas (15) and american (high 40s). again, as much as ratings sucks, i think tier 1 is a lot like tier 2, in that it's more broken down- firms look first at t14 (and further break this down, as discussed), then at t20/25, then somewhere around t35, and then the rest of tier 1 (the t35 thing is a rough estimate, but it tends to represent the schools within tier 1 that stay within tier 1, while the rest of tier 1 can be pretty mobile, often slipping out of the tier altogether).
of course, as you've mentioned, it depends on the firm. some won't touch students outside of YHS. others won't go outside the t14. however, i think many firms still draw a big difference between, say, texas or UCLA and wisconsin, and also between wisconsin and somewhere like alabama.
Yeah, I think this is a pretty accurate breakdown of Tier 1. It's probably more helpful to think of progressive "clusters" rather than a strict hierarchy. In general, I think a useful guide would be:
1) Trinity (YHS)
2) Elite (t6)
3) Superior (t14)
4) National (15-34)
5) Strong Regional (35-60)
6) Regional (60-90)
7) Borderline (90-100)
Hard to say, though. Some schools move between categories, so I think the best way to use the rankings is in mapping out, at the very least, five-year trends.