Regardless of how powerful an agent machine gets the number of virtual users in the agent capability matrix never goes above 100 for browser-Driven users (shown as 50 while agent resource utilization is at 50%).

There is no cap on the amount of users any machine can run as long as it has the physical resources to support the load.  The value of 50 comes from an agent capability calculation which is performed when installing SilkPerformer however the formula is such that 100 is the maximum achievable value for any hardware.  It is possible to modify the CapabilityComplexities.xml file to let SilkPerformer run more users per agent.  It is important to note that changing this value upwards does not mean the machine has that capability, just that SilkPerformer will try to scale the load that high.  

This file is available here:

C:\Program Files\Silk\SilkPerformer 9.0\AgentPools\CapabilityComplexities.xml 

Regardless of how many users you decide to run it is important to monitor the machines for health data to ensure they stay within acceptable limits as failure to do so my skew your results.