I dont think there are any pros to mounting a dot scope other than a JPoint sight on a slide. (jprifles.com)
Sticking a tube sight on a 1911 slide is going to slow the slide down, screw up the guns timing, and beat your scope to death.
Get a scope mount and mount it to the frame so it's just above the slide, and you'll be fine. This method won't let you lose sight of the dot during firing either, since it won't be moving back and forth and up and down some with the cycle of the slide.
You'll just see your dot rise and fall some.