You can improve cooling performance even more by removing the P4 heatspreader altogether. It's there for a variety of reasons, but improving cooling isn't one of them
It could help but you may crush P4 CPU die because there is nothing protecting it anymore. AMD CPU has 4 rubber pads to prevent too much pressure being put on.
So how are you going protect your CPU now? pads? it would have to be of the perfect size and have enuff spring to avoid too much pressure being put on it. HS for P4 adds so much pressure on the CPU without the spreader it would simply just crush your CPU. What I am trying to get across heatsinks are designed to work with the heat spreader and without it you are doomed. You would probably also need a custom HS too for this to work effectively.
Back to the main point it seems lapping does work. I suppose it gives it the extra flatness to transfer more heat effectively and also you are actually making the heat spreader thinner it some parts so allowing for closer contact with the actual CPU, instead of its solid shell.
I won't do it because it voids the warranty, but I give it a year or so and will certainly have a go at it.