Hi Ram,
The 900-10B's stated MAX IMIX throuput (from the latest sizing guide, which I have confirmed is still accurate as nothing has changed) is 150 Megs, with 75 Megs being the "Recommended" average bandwidth usage. They go on to define this as:
"Typical bandwidth range, at 70% peak CPU load with complex policies, 40% SSL, ICAP, content filtering, the average over any time interval. The administrator can configure the ProxySG to either bypass access logging and limited streaming content."
This tells you the 900-10B can handle ~30Megs, (40% of 75) and in my experience it can do around 60 (but not much more).
Remember:
Encrypted traffic is on average 55% of a given links' overall traffic and expected to rise 10% YoY
Symantec does sell a dedicated Decrypt Box (the SSL Visibility or SV units). The smallest reasonably priced and supports 250 Megs of encrypted throughput. This completely takes the decrypt load off the Proxy (and is easier and more granular to configure encrypted traffic management).
Hope this helps