Not sure what a "BC" is, but to the best of my knowledge, there is no aftermarket locking or limited slip differential available for the 718s. If you want it, your only option is Porsche Torque Vectoring (PTV), which also requires PASM or PASM Sport.
If you have a manual car you might be able to fit a Guard LSD who make one for 987/981 series manual cars. I think there is probably no difference between the manual 981 and 982/718 drive train. However, there is no aftermarket LSD option for 981/982 PDK cars with open diff.
Yeah spoke to a local specialist, he’s never done one but pointed me to Guard. Price of the unit for a 997? was $3300. My plan to do a base cayman and add diff ($5k) and add Ohlins ($5k) was surely stupid. I’m now concentrating on a B or C T.