I am not very familiar with TwinCAT, but it seems like the overall real-time capabilities of the system you are describing come down to the capabilities of the TwinCAT software.
My understanding of your workflow is that Real-time workshop (or Simulink Coder/Embedded Coder in newer versions) produces code that is then compiled through TwinCAT and run on the TwinCAT real-time system. The amount of work that can be in a given time step would be dictated by the hardware you are using and the quality of the compiled code coming out of TwinCAT. That being said, there are also optimizations that could be made when generating code using Real-Time Workshop that would make it go faster as well.
Really, though, this would probably be a question best posed to Beckhoff to see what TwinCAT is capable of.
