One of the problems/challenges with using lower temperatures with rads is that even though 35/36% rad output might be perfectly suitable with flow/return/room temps of 45C/40C/20C is that if you switch off the heating for say 9 hours at night then it will take forever to get the room temperatures back up after heating back on in the morning, gas/oil fired systems can/will run the rads at ~ 100% output for say 1.5/2 hrs to get the room temperatures up and then via TRVs or whatever reduce the outputs back down. Obviously increasing the rad sizes by a factor of 2.5ish will largely deal with this but probably still won't get the very rapid heat up time as the convection/air circulation off the rads is much slower at the low temperatures.
Hence setback.