As you suggest, yes radiators work 'mainly' by convention, but the also emite via radiation (somethign like 80% convention 20% radiation) and these reflective panel have proven effective when a) used behind rads on outside walls to minimise heatloss via the wall, but the main area of loss is the 4-6 inches directly above the radiator, and most would find the foil visable here unsightly.
Whatever type of reflective panel you fit, ensure there is always a minimum 35mm air gap behind your rad (especially on a single rad) to allow
for the convection current, there should be at least 100mm clearance above the rad to any shelf/ledge for the same reason, and avoid curtains that hang above the rad trapping theese convection currents