Kind of fixed. There's some random element involved in the quality of the ASICs you have but that can't be changed after manufacturing. As a result, I'd bet the operation modes currently provided in the Apollo software are safe (or worst case) averages to ensure the software works without issues with almost all units. The variables that can still be changed are:

• voltage
• frequency
• temperature
• bfgminer
• other parts of the Apollo software

Playing with the voltage and frequency values could provide some unit specific efficiency or speed gains, as was the case with the Moonlanders. Temperature is obviously also part of the equation if you go for maximum speed. Since bfgminer isn't static, some new version could theoretically improve something. The other parts of the Apollo software are mentioned mainly from efficiency point of view as the MCU also has some consumption but it really doesn't matter when compared to what the ASICs need.

I'd guess some future version of the software could include some kind of autotune for the voltage and frequency values if speed or efficiency is the target at some specific hash rate but I don't see that as a priority and there hasn't been any discussion regarding such being even in development.