yes, if there is only one SGSN instead of 2, then you should see less inter-SGSN RA updates. But the number of RAU (inter+intra) should be the same as before.
The RAU is a procedure starter by the MS. The MS doesn’t care about your Gn interface… it just cares about the RA in the new cell : same or different than before ?
If different, then a RAU is done.
I’m not familiar with interSGSN RAU : are they going through the GGSN ? I thought the SGSN would contact the other SGSN directly… (to be verified !)