In Civ IV, the tax/research slider was for ALL cities.  Also, there was less percent multipliers.  The most you could get in the early game was a library, giving a +25% research multiplier to the city.  

THE GAL CIV III ECONOMIC MODEL IS TOTALLY DIFFERENT.  Not only can you have a separate slider for each planet, but from turn 1 it's possible to build MULTIPLE +25% research multipliers per planet AND surround it with starbases that provide multipliers to RAW PRODUCTION.  Consequently a planet can EASILY reach +300% research or more by turn 20 or 25.  

Civ IV was mostly balanced by the game mechanics, restricting empire growth to LINEAR increases, Gal Civ III's economic model, thanks to specialization, is EXPONENTIAL.  

Consequently the normal tricks that developers used for balancing Civ IV economic model WON'T WORK FOR GAL CIV III.  Either the AI gets TOO LARGE OF AN ECONOMIC BONUS giving it too big of an advantage in the early game or the AI gets TOO SMALL OF AN ECONOMIC BONUS making it inconsequential in the late game.  

Once again, to repeat, thanks to Gal Civ III's exponential economic model, giving the AI flat bonuses ISN'T GOING TO WORK.  

In other words, the AI needs to be taught how to specialize its planets.