Depends on the rad, if it is a plain one then it will not make much difference, if it has fins on the back as most do then it needs the flow of air to allow it to perform at its best.
Also if its got a TRV you would be better changing to a standard valve as the build-up of heat in the cabinet will make it turn off too soon.
And don't forget to paint everything inside the casing matt black for the best visual effect.
Jason