Not much!

It seems people are trying to make the case that higher pressure under a wing is just as responsible for lift as lower pressure above a wing.

Of course pressure differential is required to produce lift but the upper surface of a wing is most critical. That airflow over a wing produces a pocket of low pressure above it and the wing moves to fill that void. It's like it gets sucked up into the area of vacuum just like the AF sign says. Destroy that pocket of low pressure and you fall out of the sky.

At work (I'm a corp/charter pilot) we have operational specs that state we must remove any ice or frost from the wings/tail before flight. I cannot even polish frost so that it's smooth. It must be removed. That's how critical the top of the wing is. If I have frozen condensation below the wing (usually near the wing root where the fuel is sits and it's below freezing) I do not have to remove that. It's just not as critical.