Rubbers are only part of the issue. Like RET correctly says your are pushing what amounts to a brick through the air so you won't get it 100%
That said, my FC uses multiple sound deadening layers and is actually very smooth and quiet.
Under the car uses spray on sound deadener, as does the internal floor and inside the door panels.
On the inner floor I then use a sound proofing sheet, the same type used to insulate the cabins in earth moving equipment (from Daley's).Then I use the traditional cotton wadding underfelt and then carpet. All layers extend up to and slightly beyond the factory inner firewall insulation, and under/behind the back seat.
I've removed it now but was also using an under-bonnet sound insulation pad.
All rubbers and bailey channels were replaced with Rares parts, as was the exhaust hanger rubber.
Radial tyres also help with overall NVH.
The only noise remaining is generated around the 1/4 vent windows and sunvisor, and only above 55mph. Below that speed its as quiet as my new Mazda. Pump the front tyres up to 40psi and its also got pretend power steering
So in my opinion it can be improved over factory but you need an overall strategy.
