An update - I transited through BOM last week and had a great experience. Arrived by Lufthansa (thankfully flight was on time) and cleared immigration in record time. Suitcase was already waiting at the carousel when I got there. Dropped off my bag, which had been through booked to Chennai, at the Jet transfer counter. In a total of just one hour after wheels touched down, I was waiting at the gate for my Jet flight.
I'm really impressed by BOM compared to the lethargic speed of international arrivals processing at MAA. BOM seems to operate at double speed!