What is the spec for the exchnage server, the best thing to remember is that 1 BES user = 4 Outlook client users, so if you mutiply your 600 BES users by 4this is the equivilant of having 2400 exchnage users added to your system.
There are also other factors which can affect mail delivery delays, the most common is the AD/Ex2K3 permission setup. Which is covered in detail in earlier threads.
Another issue which i have seen and get is memory leakage, which is referenced at A third-party MAPI program stops responding, and Exchange Server 2003 intermittently does not deliver messages
and relates to emsmdb32.dll problems with versions prior to6 .5.7233.36. This is an Exchange issue which affects 3rd party MAPI apps, which may also be affecting the system.
The specs on your BES server seem o.k for the number of users you have.
I would recommend that you run some perfmon stats on your BES server and monitor the memory. Log it to a .csv file and you can create a graph form it in excel, this is good to see how the memory on the BES is doing as if you find it starts to nose dive and does not recover, this will cause issues.