I've been looking around here and ingame, and I can't seem to find the calculation of how fast BRL is when loading/unloading wagons. I know it scales with stack inserter bonus research, but I don't have a baseline from which to calculate.

In particular I'm playing krastorio2 and at this point I have the regular stack inserters and I want to know how many of those compare to a single BRL

Without any research, they (un)load at 120 items/s. At full vanilla research it should be 720 items/s. Infinite stack inserter hand size research, if added by another mod, allow them to scale infinitely (up to the stack size of the item being transferred).