I just played the Trebia battle with all the Roman maniple infantry and the Carth heavy poeni inf set to experience 8. I did increase the size of the Carth poeni to 90 men and the barbarian inf to 80 men. I made all skirmishers on both sides 80 men. I left all the cav unchanged except I inadvertantly reduced one sacred cav from 60 to 40, and didn't change the elephants. When I played this modded battle at medium difficulty the infantry fought for a long time, the casualty rate seemed low and cav did not plow through them. The two sacred cav in the rear were actually blocked by two triarii. The importance of the elephants in achieving the win seemed increased, although, one got spooked and almost cost me the battle, and I had more time to give individual unit commands. So, it played a lot slower as far as fighting speed (rate of casualties), and the cav couldn't blow out big infantry units, but the infantry was pumped up a lot higher than the cav.

I then played with all units set to exp 0, weapon 0 and armor 0 at CBR's suggestion, but the battle was over very quickly with a mass rout of all the Romans once the elephants engaged. Total casualties were 1/2 what I got with the higher exp modded game (700/200 with exp 0 and 1400/400 with pumped up inf). This shows that morale is still tied to the experience upgrade.