PSA: No dealer margin on these so we don't sell them, but we're using them in our high hp NA/NB builds using the 7" Miata diff.
Driveshaft Shop P/N RA6248L2, same left and right. Don't ask me tech questions, ask DSS. OEM shafts are all rebuilds. Failure mode is usually the bearing race brinelling which causes them to bind under load and pop out of the diff housing.
Takes lots of torque for that to happen. It's just a matter of time with OEM axles. Andrew shattered a tripod doing a 20mph clutch drop at 220whp during an outlap in his S1. Gordon's 450whp Rotrex build ate halfshafts like free chips and guac at taco tuesday.

interweb $359-399 (each) + 50 for ABS tone rings if you need them

 

Any clue if these can be ordered with the high hp hubs as well?

 

Looks like I might need a set soon.

 

They also have these for msm diffs now.

 

V8R makes a larger billet hub. I'm sure you can have the larger end put on, but it wouldnt be an off the shelf deal. I've still sheared one of those bigger ones in half, so there's that. YRMV.