Copper has less resistance than lead. Just look at all of the lead that gets ripped off of the bullet when it goes down the barrel. Jacketed rounds don't have that problem. Besides, if jacketed bullets have more resistance, then why don't we need to lube them? Lead bullets get lubed because leading would be ridiculously high without it. While you may get higher velocities with lead at the same powder charge, you will almost always be able to push the jacketed rounds to higher velocties because in order to push lead that hard, you'd have to send the pressure beyond the safe zone. Whenever I look at a load book, I can find loads for jacketed and lead of the same weight and jacketed bullets max out 200 fps faster than lead in 9mm.
The reason you get higher velocities with lead is because of the resistance that it gives, just like you get higher velocities by tightening the crimp. The resistance allows pressure to build. This also explains why jacketed rounds can be pushed faster, they aren't reaching maximum pressure as soon.
Remember guys, less powder does not always mean less pressure... especially when you're changing other components.