Actually Karachi - Bombay is 870km, greater than the Sargodha Agra. But thats beside the point, because the IAF actually mounted raids from Pune to Karachi using its Canberras - which was a distance of 1000+ km. But then most of the flight was over our territory, so IAF could use the Hi-Lo-Hi method. The PAF would have had to spend a bulk of its flight time over indian airspace to get to Bombay, and if they went in Lo, they would be at the extreme range of their striking power.
The PAF staged its Agra raids from Shorkot (680km) but then it could muster only a two aircraft sent in single ac strikes. Night raids are difficult to intercept, but they are also difficult for the attacking crew to make any impact. Thus inspite of the easy landmarks, the taj shining as a beacon in the moonlight, the Agra raid didnt achieve much.
Even in 84, only the B-57s had the night bombing capability (navigation by night, bombing computer), the Mirages would have had to use their eyeball mk1 to locate and strike at the dome. good luck doing that by night. especially when you have an SA-2 installation watching over Trombay
right across the bay. (Did i mention the AA guns?)
Doing that by daytime, you run the guantlet along Jamnagar as well as Pune.
I am not sayin the PAF cannot muster a bombing force - but that its an incredibly risky thing for them to do , with the chances of success being very low.