Dear Kr,
As long as I know, IMSI is restricted to send in Air Interface (please CMIIW). So if the MS is still on the same LAC and CI, the MSC will only sent TMSI which is the encryption from the IMSI. IMSI will send if the New VLR doesn’t know where is the Old LAC/CI because there are not in their dbase (EXTERNAL LAC).
Dear SPB,
IMHO perhaps your decreament in Paging Success Rate because the subscriber had turned down their MS in that time, so there are no paging response from the MS.