Some ships will have to use thermal spread for sure, 20% increased heat is a lot, too much for ships that already hit ~80% heat when charging near a star or within planetary gravity.
Not had any issues on the ships i fitted the FSD++
But my jumpers usually run heat efficient PP - either armored or low emissions (0.13-0.35 efficiency). At worse i use overcharge 1 with thermal spread (0.38 efficiency).