As for heat, one needs to consider rotational speed. On motorcycle wheels, it's comparatively slower than electric motors. I'd say some extra grease is beneficial in a slow turning bearing operating in an abusive environment. Dirt, dust, silt & water to name a few contaminates. I'm also a fan of synthetic lubricants as many advancements have been made in grease since lard was used on wagon wheels. Stay vigilant and ride safely. My 2 cents.

I would agree that speed/load (internally) generated heat is a non-issue in most motorcycle wheels. The seal keeping the crud out is what's doing all the work. Prying the seal open to add grease seems very counter-productive.

I just replaced the front wheel bearings with sealed SKF units and never had any problems (OEM grease was fine). I forget the bearing number but it's standard and easy to get. Our rear wheels got modified with a billet machined aluminum hub which had a grease fitting in the center. The bearings where sealed on the exterior. Kept up with the grease and never had any issues with those. I think the stock rear wheel bearings are also a standard bearing number. I'd replace them with sealed SKF units too. McMaster Carr has good bearings. Ride safely.