I'm not aware of any tests that have been carried out re this........ there probably has been though. Logic would say 'yes', in answer to your question (my logic anyway fwiw) although probably only in a small way. Every little helps though. I have done all mine this year when I took the rads off the wall to get them flushed out properly.
Foil reflects thermal energy so I think it dose a bit, but I like the fans under seems to make it much warmer. Keep meaning to look at that
I would have a look at Radflek (think that’s what they’re called) as they have plenty information on their website on energy saving, heat output etc. Think you can also get these mini fans that sit on top of the radiator that helps distribute the heat should that be a problem, however they look a bit of an eyesore. Think Plumberparts has done a video on YouTube regarding these. Hope this helps.