I've tried the Load Balancing feature and it does indeed help spread the load and remove the heavy load from the #1 core for some intensive FX plug-ins - bravo! But, I am curious as to why this feature is not available for Virtual Instruments - per Cakewalk's statement about Load Balancing...
 
"When enabled, SONAR will attempt to load balance track/bus/clip/ProChannel FX Racks, FX Rack FX Chains, and ProChannel FX Chains if any of these contain two or more unbypassed plug-in effects.(However, please note that this technology does not benefit virtual instruments in the synth rack, FX Racks with active side chains, or External inserts.)"
 
By the wording used by Cakewalk, it emphasizes virt instruments "in the synth rack" - maybe wishful thinking on my part, but does this mean that load balancing DOES work on a Virt Instrument if inserted into the session a different way, such as inserted in a Simple Instrument track, or inserted in the FX Bin of an audio track?? Any other trick that might force the Load Balancing to work for a Virt Instrument plug-in?
 
Typically where my system runs out of steam is when I have a few Virt Instruments running, so having Load Balancing help with these would be even more useful a feature for me!
 

Virtual instruments just don't lend themselves to load balancing. The few that support multi-threading (e.g. Kontakt) handle it internally. I don't know what the current advice is from NI, but in the past they've discouraged the practice, as the overhead of managing multiple threads negates most of the benefits and could lead to instability.
 
For myself, I find the CPU load for synthesizers to be insignificant compared to heavy-duty effects such as reverbs. Sampled instruments are another matter, of course. But even then the bottleneck is almost always I/O, not CPU, and SONAR's load balancing feature won't help with that. The only option you have for mitigating I/O bottlenecks is to implement your own load balancing by reading samples from a separate drive or from multiple drives.