I am getting exactly the same - between 30 % and 50 % dedupe for full vmdk type 3 backups with non-optimized - average is about 40%. I will be installing 6.6 after Sep 14 and will update the new dedupe figures.
Flippie, are you using optimized or full in the vmdk type settings in your policy? It would also be interesting if you could tell me how much deduplication you are actually getting on the original full size of the backup. I am about to setup exactly ...