BSNES core is not implemented yet on the RPi (I dont see the option to install the core in RetroPie Setup), the only snes emulators inside retroarch are the following:
PocketSnes
CATSFC (experimental)
ArmSnes (experimental)
…but maybe something was updated, I’m going to check this. Btw, even if BSNES could be implemented in retropie dont expect full 100% speed, not even half of it, BSNES requires a high end PC to run in accuracy mode, I doubt the Pi can handle it right now.