[SoCal LS-L]

All cars accelerate slower before they are warmed up, as engine management only gives the most efficiency after normal operating temp is reached. Also with older engines, the piston rings seal better and give better compression after the heat has expanded the engine parts. Id suspect more of a sensor such as the coolant temp sensor. If your exhaust has holes before the o2 sensors...that will make it run too rich and hurt your acceleration after it has warmed up.

For the exhaust...you want to run just a straight ricer muffler out of the rear instead of the straight to dual setup?