I don't think there has been any major change in 4.2.3 compared to 4.2.2. How exactly are you measuring the performance and what kind of performance?
I've a product, which is having a SLSB.
I'm measuring the time taken by the call to invoke any operation on this SLSB. My client is based on java/RMI.
And, as mentioned earlier; my later investigations suggest that latency is poor because of "all" configuration.
So, I agree it has nothing to do with JBoss 4.2.3.