Bad news, folks...
I'm starting to run some profiling with transactions enabled, and it looks to me like there is a memory leak somewhere.
I'm commiting every 500 transactions, adding 150 bytes at a time and doing a ton of inserts, and the heap space looks like it just grows and grows...
Haven't figured out where the memory is going yet, but this is going to be a problem in a long running system.
I'll post more data as I get it.
Bahh... please disregard my prior posting. I was actually testing with large chunks of data, and I wasn't commiting frequently enough.
I'm still concerned about the performance of the transaction manager, but I'll dig into that separately.