Do you figure you'll lose much benefit from not having the antenna above the roof line? I seriously thought about adding a WeBoost too, but I'd be in the same boat as you with fitment below the roof.
To be honest I will run tests when it is all said and done. I have yet to install the QPM and run power. Depending on signal I just might add the other extension.
I know of someone that runs his antenna on the front bumper of his rig and has great results.