the pinion angle is one of 2 key angles, the other being the transmission angle. U-joints dont like a lot of angle for one. and if the angular difference between the trans to shaft, and shaft to diff joints is more than a few degrees, you get noise, vibration, and joint failure.
This does not apply to the GTO in stock form. one, we have and IRS, that doesnt move much. two, we have a 2 piece shaft, which by adding a middle joint increases the variability (althought the angles between joints on the same shaft must remain within the tolerance). And 3, we dont have a pinion u joint. What we have is a rag joint.
These angles matter most on SRA vehicles. They can be alltered by lowering or raising the vehicle, or by cargo loads. They can also change on a momentary basis during acceleration. This is called axle wrap. The pinion will move up, as an oposite reaction to the wheels moving down. What can I say, Newton knew his shit. Motor/trans movement can also change these angles, either thru torque rock, or mount bushing failure.
The closer you get to zero difference at static position (all 4 wheels on the ground, full vehicle weight), the more balanced the angles will remain under momentary dynamic conditions.
In SRA vehicles it is adjusted by changing link lengths on multi link suspension, and with wedged shims on leaf springs.
On an IRS like ours, youd first have to limit travel of the x-member the diff bolts to. There is some play there. Pedders subframe bushings and a HArrop cover will do that. to point the pinion more up, put shims under the Harrop-to-body bracket, or above the differential's back 2 dif-to-xmember bolts. to move it down, you shim the front two bolts.
These angles do matter on a 1 piece DS IRS car. as i said, the diff xmemeber does have play, as do the engine/trans mounts. You dont have as much issue as a SRA car, but it is still there. The chances of major noise and vibration are low, but joint life is still a factor.
also, you cant have a zero angle. the bearings in the ujoint must turn, or they will lock up. this is why old chryslers offset the engine to one side, and why GM engines slope to the rear. Ford cants some of theirs slightly. No i see people saying 'but you cant have a zero angle anyway, for the reasons you listed", and youre right. But thats momentary stuff. The car spends most of its time at the static position, be it idle in a driveway or parking lot, or cruisin downt he road. This is when the joint would seize. It would then fail during a momentary change. This is why its still important to check an IRS cars angles.
The exception is a 100% hard mount, like a c5/c6 vette. Those cars do not have a DS, they have a Torque tube. The diff is always in the same spot, and is infact directly connected to the engine. it does not move. This is how all IRS should be, IMO.