Dynomat only works if you do the whole door. Not just part of it. It's also only benifical if you have subs and all new speakers as well.
Any is better than none.
Not sure what you're saying as far as the sound system goes.
Sound deadening will help quiet the car and make the system SEEM louder, as there is less background/ambient noise. It will increase sound quality as well, but certainly NOT only beneficial if you have a system.
Doors normally are the first thing to hit, then firewalls and floors. You don't need to cover the entire panel to see difference, but obviously it is preferred.
There is a lot of material out there, that works just as well as Dynamat, for a fraction of the price.
As small as our vehicles are, you can cover all untreated surfaces for $100-200 depending on which material you choose.