It's unlikely. A better speedo will not get as hot and will therefore be more efficient, but the margins are tiny with a brushed motor. You'd be better off buying a trayfull of silver can motors and testing them all on a dyno to find the most powerful. That would give you more performance increase than changing the speedo.
The other thing that will give you more go is carefully preparing the silver can motor. There's a thread
here that is very long, but can give you some hints and tips. HTH
