If you decide to go that route, dont leave the domestic terminal at BOM - there is a (almost hidden) sign for the inter-terminal bus right by the baggage carousel. The ride takes 15-20 minutes, but I dont know the frequency of the bus. It drops you off ouside the international terminal, so you will need to go through security again.
+1..
to the OP-- check the transfer to BOM thread in the AoI forum and you will get many details there..