Will upgrading to Lion boost performance and responsiveness?
All of the press / industry coverage is focusing on the huge usability changes (and that mirrors the marketing that Apple is doing).

Some notable exceptions are the new Safari which runs some benchmarks like web app and JavaScript significantly faster.

The major changes under the hood speak more to security, sandboxing and plumbing the new multi-touch interface, encryption, auto save, auto revisions rather than speed.

It is too soon to tell if speed is much better, but once people adapt to the changes, the idea is we will be much more efficient with the computer waiting for us less than the other way around. I would expect your experience with the hardware you mentioned to be a minor speedup in pure numbers but a major speedup as your brain becomes free from worrying about saving, window management and revisions (assuming you adapt to the changes and like them)

I get the feeling that with mission control and full screen apps, that the UI is incredibly fluid and optimized but I'm running on some faster macs than yours.

Fun times, huh?