what turbo is on it, that sounds like my race car which hit's boost then the waste gate and boost control kick in, however that car boost hard and normal.
but the other thing I can mention is my road car that does the same thing hit's boost hard in 1st and second then in third it feels like the nachor has been thrown out, so maybe when it's fully on boost the car is supplying the power at a constant rate and you don't get the massive accelration feeling.

Las t option check the aftermarkket BOV and make sure it's on good and tight and the air isn't leaking out around that gasket when boost is prolonged.