Good question...Depends on the type of spa. Some of systems like the Balboa M7 have sensors at the entry and exit of the heater and look for a difference in temp between the two however the sensitivity of the probe is what you seem to be asking.
The older thermostats might do that but you can't really set it. Most of the current systems are accurate within at least one degree.
So the answer to your question is no...you can't calibrate a large difference between your thermostat and your temp sensor on most spas without causing failure messages.