We only have maxed out at around 1200 users on a single SA6000 only doing SSLVPN (web/file rewriting) and no NC.
3400 is less than the advertised limit on the SA6000. We have seen in many cases where the admin console shows 100% CPU but the performance of the box is just fine. JTAC has told us the CPU % was being displayed incorrectly due to some bug. Apparently there was a bug in the web rewriter causing this. So perhaps the 100% number is not accurate unless you are seeing significant performance impact.