Well, ok where to start...
First off, I am not convinced that the MO has speed sensitive steering. It does have speed sensitive audio volume control
. There is nothing in the SM that makes me believe the MO has it. It's an oil pump with a vane rotor and a reservoir and a flow control valve, pretty standard stuff. More over, speed sensitive steering pumps apply more power steering at lower speeds than higher speeds, and well I just don't feel that. But maybe it does and I just don't feel it. Maybe it's rpm dependent and with 1800/2000 rpms at 70 that's not too far off idle anyways. In which case if thats the way it works then it should be called rpm sensitive steering not speed.
Early models had a problem with hard steering at low speeds, I think Dennis Fink had the first damage due to this issue. But I have not heard of this problem since then. I haven't confirmed it but I believe there was a change to the pump for added power. Interestingly I never had this issue on my 03. So it may have been a mid year change or just a bad batch of pumps.
So yes, it could be the steering pumps are now too powerful where before they were underpowered. But I don't feel that it's that much different at lower speeds than my 03. Whats different to me is at highway speeds. I guess I should add that it doesn't seem to wander as much at 2600 as it did at 5. My bet is that means it tires in my case.
What was common was the steering rack bolts are not tightened properly, there is a TSB on that but I thought they had that fixed in these later models. But that can cause what both of you are feeling as well. Usually though you hear a clunck or bang when you rotate the steering wheel from pin to pin. Oh note here, the SM says do not hold the steering wheel at either pin for more than a few seconds. It says the pressure goes high and the fluid heats up quickly.
I too feel my 06 wanders more than my 03, but at highway speeds not low speeds. I have been thinking about this. It also isn't as stiff. Just little things I have been wondering over, not major. The stiffness would be shocks/springs not relevant here, just differences I noticed. Pretty tough for me to tell though because there was a 5 month hiatus inbetween where I drove the Z and its really stiff and really tight.
More likely though, this wandering issue is probably caused by alignment or tires. At my 3000 mile oil change there are 2 things I have on my list currently for them to check. One is the alignment because of the wandering. The second is a cyclic noise from the front that is either a tire problem or a CV joint problem. In my case, it will probably end up being one bad tire, as I have my eye on one that has way too much weights on it. It could be causing both issues. I guess we'll see.
In addition to the loose steering rack bolts issue, another common problem on any year model is damaged suspension components due to the carriers that trucked the cars to the dealer. This is a notorious and persistant issue that comes up occasionaly but seems to always come up if you know what I mean.
In your cases I am betting that it is alignment. Have them check all four wheels. But a close second place is tires. I have had tires that make a night and day difference in the tracking and centering/handling.
It could also be a steering pump/belt issue or rack issue. In any event, continue hounding your dealers.
