The Shaker 500 factory stereo has a volume/speed control which automatically increases the volume as the speed increases. when I change the Z! will I lose this feature????

Yes since the z1 doesn't have that feature. Which if you think about it for a minute doesn't make sense. The higher end headunits have speed control volume. You'd figure the z1 it would be even easier to do since there is a speed sense wire. Ahh whatever.....

pioneer would almost certainly have to make it adjustable though, like for some cars the raise in volume would be much greater then others.

Of course. It's adjustable on thier high end radios. I forget what the setting is called though. SCV comes to mind though I could be thinking of gms.