the custom firmwares are not designed to change sync speeds. So sync speed is irrelevant.
If there were extra options in the firmware to allow you to change sync speeds (or at least target snr levels and interleaving) then sync speed might be even more relevant.
As it is they are not designed to change sync speeds. But
there is strong evidence that different firmwares do do something to the way syncing happens (I'm not sure what) that means you achieve different sync speeds (and different interleaving) with different firmwares. The main reason (at least for me) for special firmware is monitoring, and the main reason for monitoring is to make sure you are getting the best tradeoff (for you) of reliable connection, fast sync and low latency. So sync speed isn't irrelevant.
Moved (with trepidation turned relief) to BT Infinity 2 for upload speed. Happy BE user for several years.