.40 Super is more powerful than 10mm, so I hesitate to put my RMR back on my FNX after the conversion.  Anyone using an RMR when shooting .40 Super?

I did not know EFK made a .40 Super barrel.....

Would have to look it up but I thought .40 Super was right at or slightly behind true 10mm loads.

My money is on the RMR being fine.

I have and RMR that died after about 2 years on a high round count Glock 9mm.  Trijicon repaired it under warrantee, and it has worked fine since, but I still wonder if it will be reliable under the higher recoil that the .40 Super will deliver.

Underwood ammo is known for having pretty great 10mm loadings, however, their .40 Super 135 grain loading (which uses the exact same bullet as the 10mm loading) has 23% more muzzle energy and the 200 grain .40 Super load has 25% more muzzle energy than the 200 grain 10mm load.

That is a pretty dang significant difference in power and velocity.  In fact, the .40 super 200 grain load is 100 FPS faster than their 180 grain 10mm loading.