It's the same % bonus as damage boosts for swords or guns, so about 6.5% per point of damage bonus up to +39% - just over an extra bomb's worth of damage per 3 bombs.

At 4 points of damage bonus (2 Elite Boom Modules) the bonus is around 26% instead, which is an extra bomb's worth of damage per 4 bombs, which is probably more relevant if you're only getting a damage bonus from the trinkets. So you'd need one less bomb for things that normally take 5-9 to kill, 2 less for things that take 10-14 to kill and so on. There might be side cases where the damage needed before the last bomb was so low that the damage bonus cuts out another bomb as well.

The important factor is, of course, how many bombs it takes to kill something. Unless a monster is left with a very low amount of hp before your last bomb now, a single level of damage bonus won't change anything, even though you get higher numbers.

Because getting a very high damage bonus is easy (and safe) now, non-status bombs may start to benefit equally from damage bonus as they do from CTR; being able to lay 5 bombs in the time it usually takes to lay 4 is very close to being able to kill something with 4 bombs where it usually takes 5 (arguments could still be made for either being better, mind). Of course, the many chemical bombs still benefit more from CTR.

I'd say if you already have a good CTR and you want to mainly attack with bombs, boom modules are a good investment.