What is the optimal altitude for performance and mileage in normally aspirated GA planes?
First off this highly varies by aircraft. Piston aircraft that fall under the GA header these days have a pretty wide range of altitude capabilities. The Piper Archer I fly wont really get over 10,000 but you can get your instrument rating and grab a Mooney Acclaim which will take you well into the flight levels and get you up to 25,000 Ft. 


  best altitudes for ground-speed....


The simplest answer here is the altitude at which your tail wind component is the best. Wind speed and direction does shift with altitude and in many cases your [(increased performance/cruise time) + tail wind - (reduced climb speed/climb time)] is best where ever the tail wind component is greatest and your climb does not take impractically long. 

Very Generally speaking performance increases with altitude to an extent. However climb rate usually decreases so there is a break even but it for sure varies by airframe and configuration. 


  fuel efficiency...


Again this varies by aircraft as well as how you chose to operate the plane. Things like running lean of peak can effect efficiency at the cost of speed typically. Some planes have little to no change in endurance at varying altitudes (although your change in airspeed may allow you to cover more distance). At 75% power my archer has almost no change in endurance as I climb.  



Variable pitch propellors also offer a great deal of power setting options that can allow you to tune your efficiency even further.