The speedo is brighter because it's a new face. They dim a bit over time. They should be equal in a month or so. I have a dial on the converter to regulate the glow and have it set about half way. As they dim I'll turn it up. (note that at full power, the converter has a noticeable "whine". I put it far back in the dash so I don't hear it.
You could disable the center circle IF you are very patient and careful. You just purposely "damage" the gold part of the back of face like I did. Cut a neat circle around an area you want dead and either peel off the indiglo layer, or cut it off from the current.