The stock ba cooler is a pos. Gains of up to 20rwkw have been seen by replacing the cooler alone. I put a wog cooler on stock pipes and stock tune on mine and it made heaps of difference, especially to throttle response. And as for the "turbo flutter" it is slightly louder, but it only occurs when backing off the throttle while on boost and sounds somewhat like a wastegate, it sounds really good. Heres a pics to compare the stock cooler to a wog cooler