Customer complaints may decrease when you use SPDCH instead of FPDCH : the BSC congestion may decrease. Isn’t that what you wanna do ?
If your operator is SO concerned about subscriber QOS, then it should by more BSC capacity. It’s a small investment for a very high revenue.
All solutions to decrease BSC load will decrease subscriber QoS, on way or the other (except SPDCH, which is totally transparent for them)