main differnece is more memory which allowed some extra features:

in ford form they didnt offer any advantage other than the P8 offered closed loop boost control with overboost facility.

i've been told that the injector drivers are superior in the P8 and the same car with just a swap of the ecu hardware form L8 to P8 will produce slightly more power

the P8 has a Limp mode in which boost and revs are limmited if it detects a faulty input sensor. a lot of poeple slate the P8 for this saying it's unreliable - BUT its actually highlighting faults that a L8 will run with and in turn COULD end in catastrophic failure.

the P8 hardware can also run coilpack/wasted spark althoug the ford software doesnt utilise it.

with revised software from the likes of RP labs, you can have all the facilities of the P8 on L8 now, and be able to live map it - which IIRC there is nobody in the UK who has the setup to live map the P8.