I've spoken with a couple of knowledgeable and well respected sources about this.
The consensus is that the 3.0Ltr cars will not benefit from a bigger throttle body even if the more usual I/H/E upgrades have been done. It won't hurt but it won't help.
It will however benefit 3.2Ltr cars as they have slightly better breathing (bigger valves) and the cross sectional area of the standard throttle body does "strangle" the motor a touch. There is science behind this which I don't have to hand but someone else might chip in with that
Science of Speed will claim a few hp's for mechanical standard 3.0 and 3.2Ltr cars but in truth it really only comes into it's own for FI and their increased capacity (3.3/3.6/3.8Ltr) NA builds.
I plan to get me one at some point anyway
Cheers
Mark