Hard one to answer really.
A lot depends on traffic patterns as much as anything else ie. if you have a lot of traffic between vlans and these are all local to the VSS then yes you have effectively created a bottleneck by pushing them through a firewall.
Bear in mind that if most of the traffic is routed to remote networks then your uplinks also become bottlenecks although you could probably get more uplink capacity than your ASA gives you.
Of course there are obviously other benefits with VSS in terms of ease of configuration, redudancy etc. and it really comes down to your priorities.
But yes, if throughput was the primary reason for using VSS then by using the ASA you may as well have brought cheaper switches and a standalone firewall to be honest as you are wasting a lot of the capacity.
And if the customer has bought into buying VSS because of the throughput it's going to be hard to explain why they spent so much and can't actually use that capacity.
Probably not what you wanted to hear.
Of course the main issue is not the VSS but the fact that the customer wants to firewall all vlans and you will always be limited by the firewall throughput.
However if they later decide to route some or most of the vlans on the MSFC then the purchase of VSS is going to make that person look very good :-) ie. no need to purchase new switches to meet the increased demand.
So it's a balancing act and the customer needs to be aware of the potential issues beforehand.
Jon
