OK, so I'm back with a real keyboard and some time.
Pharmacist's write up is pretty much spot on in terms of addressing the Z's oil/reliability, etc.
105* exterior temps on mountain roads will likely have you right around limp mode on a 09-11, you should be fine with a '12 though.
As far the fun to drive factor-- that is up to you. The Z and S feel like two totally different animals. The S is far more connected. It transmits a whole lot more detail about what it is doing.
Even with you owning the "softest" of the s2k's, I think you will find the Z is even softer. It rides much more like the GT car it is--relative to the S, it is very compliant and a comfortable cruiser. The Z is much more prone to understeer at the limit, and generally handles "safe," in contrast with the more natural handling of the S.
Things to think about when considering the trade:
Losing shifter smoothness
Rougher engine up top
Looser connection with the car
Loss of droptop (depending on whether you get ZR)
loss of a mechanical LSD (VLSD is
)
It is really up to you whether you want to lose those things, in exchange for:
More comfort
roomier interior
notably more power
(arguably) better looks
and of course-- it's a new car
