Wow. Is there any resource on what the tiers correspond to in terms of title and experience?

You can cross-ref that with the USA Jobs site, which usually (I think?) lists the O-level of the position. What I usually do is: Find job on USAJOBS most like the one you're applying for and take note of the GS- or O- level and then go to the other site and see how much a GS-12 or whatever makes where you live: This is now your absolute floor if the company you're applying to offers the same level of benefits as the govt. (Hint: they don't)