Straight pipe has nothing to do with turbo lag. Inspect your VNT actuator to ensure it is moving with throttle application, then test it with a vacuum supply. It should start moving at 3hg, and be fully open at 20hg. If the actuator hold vacuum, but the lever does not move, the variable vanes in the turbo are seized up from carbon. This can be cleaned in a couple of different ways. Have you checked for codes?