I?ve had good success with the standard BR8ES plugs on the 8A (pMags) running LOP. The RV-10 is unfortunately still running mags (will Brad ever have the six cylinder pMag out??) and standard Tempest plugs. Even so, after balancing injectors LOP ops are smooth even when way below practical LOP fuel flow.
That said, I did have problems before I replaced the orginal Champion plugs with Tempest. 8 of the 12 Champion plugs tested bad after 300 hours (high center electrode resistance). The new plugs made a big difference.
If you are having problems running LOP and are sure your plugs are clean and in good shape, recommend you look at the other usual suspects before messing with plug heat values.
Carl