Another area to consider for sound deadening is the back of the panels surrounding the side air intakes. I am not sure of it is different for the MGF, but on a MG TF if you tap on the panel surrounding the air intake grille is sounds hollow and terrible. There is basically one large thin metal cosmetic panel that runs from the rear of the car all the way to the front of the the sill area. The area around the vent opening has very little structural support and will rattle you tap gently on it. Compare this panel with your recently sound deadened door panels and you will observe quite a difference!

If you remove the vent cover (easy to do - see other posts for this) and remove the black plastic air tube, you can access a fair amount of the back side of the metal panel (well not so much in the forward sill area unless you have very thin arms and small hands). Clean it first then apply sound deadening material as far as your arm can reach is all areas. The difference will be quite remarkable.  
Clean out your 4 sill drain holes while you are at it.  

You can also access the rear area of the panel via small openings in the boot area on the sides behind the carpet.
The front sill area cannot be accessed (for sound deadening). 

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