1st of all you can control 3 projectors and have 2 live feeds in Qlab if you're using a Mac pro. So if you don't want to apply any real time video effect on you videos or live feeds then Qlab itself will do the job. But if you want more video capabilities then you can use both softwares together!
I was exactly playing around with this idea of using both Qlab and Modul8. I've just spent a good part of the afternoon playing with this and it is very exciting!! Because it is possible to use both softwares in conjunction and use the best of both worlds (Qlab = Marvelous Audio capabilities, Precision, fine tuning etc & Modul8 = real time video modifications and effects).
Almost every command in Modul8 can be midi mapped and that's where the fun is since Qlab sends midi. :)
So you can use Qlab to run all the audio and cue up modul8 by sending it midi messages with precise timing.
I am in a situation where it will be useful for me to do so because I want to "luma key" a live feed on top of another video but I also have audio and precise cueing. The other good thing is that Modul8 now supports up to 4 simultaneous live feeds and that there are more options for capture hardware than there are with Qlab so far.
Of course, as you said, you will need to learn Modul8 which is pretty simple.
You'll need to know how to map midi commands which is also very doable.
-Then you turn on the IAC midi driver of Mac OS X.
-Open Qlab and make a midi control change message with a fade time of say 5s that outputs to the IAC midi driver.
-Open Mpodul8 and put it in midi mapping mode, select the control you want to affect. (TIP: there are 3 categories of midi mapping that you can choose from at the top the modul8. Most likely you'll want to go to the "Position" tabs. That will affect the controllers and not just put the focus on them.)
-Press Go in Qlab and then right away switch the application focus to Modul8 while the midi fade is happening. Then Modul8 will receive the midi mapping. You'll know this by seeing something like 1/1 on the controller you selected.
-Then you repeat as needed with different control numbers in Qlab, go out of the midi mapping mode.
-Set up the right midi info in Qlab and you're go to go.
Let me know what you think. And if you have more questions, go right ahead. I know I didn't right all the details of the operation.
This is fun! :)