Hi Alan,
The SME servers I'm still involved with now, are performing networking services (DHCP & DNS) and file sharing (Samba). From memory all are HP Microservers (typically the Gen8 Celeron model, with one older Gen7 AMD N54L), with anything between 4 and 16GB RAM, and 2TB to 8TB SATA RAID 1.
I haven't seen any noticeable performance change between version 8 and 9 on the same hardware for the tasks we do, presumably as the hardware specs of the servers aren't close to the minimum requirements.
Having said that, I'm aware that whilst it looks like you are way clear of any minimum requirements as well, if you are performing different workloads like DB / Web / VoIP etc... then you might be in a different situation.
Hope the above helps.
Craig.