I am very surprised. You say you see no difference on the A interface, yet you say the TMSI reallocation might cause the SDCCH to last longer.
But … you should have cleared that out by now : the TMSI reallocation procedure is visible on the A interface. Why don’t you simply check the duration of this procedure ? Maybe it does not impact the duration of the whole SDCCH phase at all ?
In alcatel system, the tmsi reallocation is performed in parallel to the authentication/ciphering/call setup. It doesn’t add any delay.