Owned my car for 5 years now. Only a lady friend asked to drive it. It sounded so ridiculous to me I laughed a little and said no. I found out she was serious when she immediately asked a second time. I just told her, I don't let anyone drive my cars.
If you want to be nice, you can always ask them what's the fastest car they've ever owned. If it's slower than the Z, you can use the Z's too fast for them excuse. If they had something way faster, ask how long they had it and if they ever hit anything with it. That still could put them in to the Z's too fast for you category. If that checks out, ask them how many people they let drive it. If everything still checks out, I would be, "yeah right, I'm still not letting you drive it."
Seriously, unless it's a life or death situation, there's no good reason to risk someone else screwing up your ride.
One exception, if they let you drive something bada$$ of theirs in the past. Then you kind of owe them. That's why I don't accept offers to drive other people's stuff.