The current absolute minimum time-step for the RT Box 2 and 3 is the same as the RT Box 1 at 1 µs. The execution time of a specific model on the RT Box may be larger than that based on the required computation of the model. It sounds like 2 µs is the limit for your model on the RT Box 1.
The best way to get a true comparison between the hardware units for your use case is to email email@example.com. If you provide a representative plant and controller model, the support team could run the model on both sets of hardware and provide an initial set of benchmarking data. They would also provide recommendations on how to further optimize your simulation model for real-time.
Also note that you can perform an offline "CodeGen" simulation to compare how different discretization step sizes can impact the model fidelity. See the "Plant Code Generation: Three-Phase 6-Pulse Thyristor Converter" demo model for an example of this process.
You can see a comparison between the RT Box 1, 2, and 3 specifications on the Plexim website: https://plexim.com/products/rt_box/comparison
. The most notable differences when it comes to the processor performance are 1) the RT Box 2 and 3 have a higher CPU clock than the RT Box 1 and 2) the RT Box 2 and 3 have 3x CPU cores to run a real-time simulation model, whereas the RT Box 1 has 1x CPU core for the model (one core is reserved for communication/supervisory purposes on all RT Boxes) .