T200 is not related to a GSM procedure. Every time something is sent on Abis, an ACK is awaited. It won’t improve the call setup time, unless you have a lot of LapD retransmissions, usually.
You can check lapd retransmission with a qos indicator.
If you update the T200 in BSC, I don’t know if the BTS is going to be updated as well. It depends on your system. Is it alcatel ? In this case, check your parameters dictionary, it should be explained there.