commitcount = 1000 should keep the logs from filling up and not hurt performance. The default log buffer size (logbufsz) is usually way to small, and should be increased if you are doing large imports (or just about any other SQL). You will need to increase the database heap size (dbheap) accordingly.
Also consider lowering the CHNGPGS_THRESH to about 15-25, which causes page cleaning (writing updated pages in the buffer pool to disk) to start sooner, which is usually advantageous with large import jobs.
If you have version 8 fixpak 4 or later, you could also try enabling the registry variable DB2_USE_ALTERNATE_PAGE_CLEANING. Setting this variable to ON provides a more proactive approach to page cleaning than the default approach (based on CHNGPGS_THRESH and LSN gap triggers).
Another important parameter is the Number of asynchronous page cleaners (NUM_IOCLEANERS). If you have multiple processors, this should definitely be set to a value of at least n-1 (where n is number of processors). It might also be advantageous to set this higher on a single processor machine, but I am not sure about that.