what size are your headers ?..2.5 mandral bent is still pretty small...your mufflers could also come into question..where do you have them mounted..test have showed the closer to the rear you mount them the better.
off the bat i would say a minimum of 3 inch exhaust..
now there are other things that could be a result in poor top end also..Gear,camshaft,Carb etc......but i would start with exhaust
