@SectorNine50:
Now I'm curious as to why this was the case between these two boxes. Can anyone give me a high-level explanation, or perhaps knows of some documentation that would explain this issue?
Sometimes there are paths between point A and point B on the Internet that have a lower MTU, and end up being a PMTUD black hole, which is especially common with IPsec. By MSS clamping, you're preventing the outer ESP from being too large for such a path by limiting the inner TCP.