Microbore limits the power you can get from a radiator when you halve the temperature drop across the radiator with a HP. So if you are already at 1m/s flow in the pipework and an 10C delta across the radiator, no amount of additional panel area will give you the same heat output with 5C delta.

You don't have to halve the DT across the radiators, but you will have extra system and running costs compared to a system that runs at DT5.