My 2001 duramax has very low power off the line when it gets hot out. Is this typical? The only modifications to date are 4" exhaust and K&N drop in filter.

whats your engine temp? do you have your ac on? could be your fan clutch engaging and robbing some power.

a loss of power under a load was what caused me to put mine in the shop and have the injectors tested .. not sure of the exact readings but at least one was off far enough for the dealership to replace all 8 (due to pick it up tomorrow)

Jim