I would suspect a leak at the turbo flange, or exhaust manifold itself, as opposed to a coupler if it feels like boost is coming on slower. A pre-turbo exhaust leak will really make the car feel lazy. Generally if it is that bad it would probably be audible, but if you don't know what your listening for it might be difficult to notice. Do you know what turbo kit the car currently has?
Thanks,
Alex Goodwin
AlexG@motionlabtuning.com