I think it has been done...one of teh 500HP+ L6 guys was saying how his exhaust pressure was lower than the intake pressure at high RPMs...
But for most of us with a "normal" turbo the pre-turbine pressure should be higher than the output from the cold side of the turbo.
Think about it, if your turbo has a higher pressure on the intake side wouldn't it normally spin the turbo backwards? AKA surge?
I have a feeling that the guys that see lower pressures on the exhaust side see it because of the excess amount of heat being put into the turbo at high RPM/pressure ranges.
That way more heat is being turned into energy to spin the turbo than just wind spinning a fan.
That is just my limited view though and don't count on it.