The maximum boost frequency will not increase, but the average frequency achieved for lightly threaded workloads could possibly be higher. It will vary by core count and workload, but the cores on the EPYC CPU can spend more time in higher boost states if the core count is manually limited in the BIOS.
For example, the EPYC 7601 processor can maintain up to 3 cores per die at its Maximum Boost frequency of 3.2 GHz. Reducing the core count from 32 to 12 per socket will ensure that the cores run at the Maximum Boost frequency.
Note that the EPYC processor will automatically boost to 3.2 GHz for lightly threaded workloads even without disabling cores, so reducing the core count will only be useful in very specialized situations.