I think you have to expect the buyer to want to drive the car themselves. I wouldn't buy a car unless I was able to drive it (although eyefi would probably confirm that I did the worst test drive in history when I bought my Supra off him, I didn't exactly put the car through it's paces, lol).
Also for many people this will be the first time they've driven a Supra, so they'll almost certainly want to get a feel of the driving position, etc. So with regardless to your 'if they ask to, do I floor it?' comment, I think you're going to have to accept that you will have to let them do the driving.
Suggest you find a good bit of road nearby that you can include on the test drive where they can floor it to check performance and turbo smoke, etc. Then if you make it clear that 'there's a fast bit coming up' it's implied that there is a set time&place for them to do that bit and that the rest of the testdrive should be conducted at a more reasonable pace. Afterall, it's still YOUR car until they buy it.
Insurance - tricky one. I'm not sure how to approach this myself. I think you just need to be strict with people and state that there are no test drives without proof of insurance. Having said that, I don't think many people are covered full comp for other cars - I know mine is only third party for other vehicles. Now if the potential buyer smacks it on the test drive...
All issues I've been pondering on myself recently.