I have an ill-informed theory as to why FCPX behaves much better under Snow Leopard. The auto-save function baked into Lion uses the Core Data layer within the OS. Core Data takes care of both the undo and auto-save function and marshalls data to one of several persistence layers. FCPX uses the sqlite persistence mechanism, which you can explore if you copy and *.fcpproject and *.fcpevent files and open them with an sqlite browser. This layer also manages the coherency between the state of objects in memory with its persistent counterparts on disk. Perhaps the memory leak in Lion is due to "unhealthy" relationships between the RAM-resident objects and their representation in sqlite.
Snow Leopard; however, does not have this auto-save functionality built-in, so the developers must have had to provide the FCPX auto-save functionality using a different method, one that is apparently much more reliable than they way they are doing it in Lion.
I'm sure this doesn't help anyone with their specific FCPX issues, but I offer it as a software developer and video editor trying to guess as to what could be causing the gnashing of teeth due to these issues. I said ill-informed in my first sentence, because I'm not a Mac developer by any stretch of the imagination.
As soon as I start seeing mischief with FCPX in my project, I'll seriously consider downgrading to Snow Leopard from Lion, no matter what the cause, as long as late-2011 model MBPs work under Snow Leopard.