At the risk of not answering your question, I'd say you need to look at more than CPU % busy to determine how loaded the filer is. How busy are the disks, what about the network? The CPU is really just one of numerous potential bottlenecks. What really matters is the latency from the client side (assuming you're serving some sort of client workload here).
The other issue with looking at CPU % busy is parts of OnTap can make use of more than one CPU.
Good question and this comes up every so often with the "any" counter. I wish the any cpu counter wasn't there... is any cpu busy at any percent. sysstat -m and -M are useful to see per cpu core and domain stats... for a single value, average cpu is more likely what you are looking for.