Unless you have an XADataSource implementation for SAP DB, you are not using xa transactions, "XADataSourceLoader" notwithstanding. In any case this problem is unlikely to be related to whether or not your tx are xa.
Have you checked the SAP DB docs to find out what they think read committed means? You might want to do a little experiment with some other dbs to see if it is a SAP DB problem.
I have the same problem with SQL Server, so I don't think it is SapDB... It have to do with my configration:
I have no direct experience of ms sql server, however a friend told me recently that their idea of transaction isolation seems to involve more locking and waiting than he was used to on Oracle. I haven't used 2.4.4 for a long time but your config looks ok.