Just installed client package
Setting Transer->compress and compress level 9 produces the command line:
"C:\Program Files\cwRsync\bin\rsync.exe" --compress --compress-level="/cygdrive/C/drl/source" "drl@host:junk"
e.g. compression level value is not inserted.
Cwrsync 5.0.0 patch 10 containing client GUI version 5.0.1 solves that problem. Check support downloads area.