Now with two different tickets you always run a risk. Having said that Jet Airways is probably the most reliable airline in India and their on time performance is based on my personal experience not that bad. Your biggest wildcard is likely BOM airport, which is operating at or above capacity and thus delays are frequent, especially when bad weather hits or one of their runways is down for maintenance. Please also keep in mind that going from the domestic to the international terminal is about a 15-20 minutes ride and you can not do that airside.
S