Can you pack bearings with too much grease?

Depends-if the grease is too thick-sticky making the bearing sluggish.

If you really pack them-use a fine grease that still has the properties/protection you need.

In cup/cone wheel bearings-I pack them fully-probably wasting grease.

Checking for play is a pain with over greased hubs with top end grease as the will still turn if overtightened.

Just don’t go mental.

Whats the application?

loose or open bearings I always pack full. Sealed bearings are just left as they come from the factory

They are the bearings on my HT2 bottom bracket. I had a Superstar BB on for a year and it spun round very freely… until the bearing started to fall apart at the weekend. So I put the HT2 BB on because apart from the fact that it was a bit sluggish it was still in excellent condition.