I believe Stelarc has added arms and ears but not amputated anything. He has speculated on the worth of the wetware we are born with and whether or not we might be better off replacing it with hardware.
You said “My main concern is I guess to suggest that maybe there should be boundaries to human behaviour, values if you like.†– but I wonder whose values?? I suspect John Howard’s recent comments are prompted by similar thinking. If there are to be boundaries – who defines them? How do we achieve concensus on what shoud be regarded as “normalâ€?  Where would such thinking have left the transgendered, intersexed or androgynes?
The film Dogville examined the notion of “arrogance†of thought… and seemed to suggest that none of us can escape it. We all seem to think that what we personally can allow to sit comfortably should define the rest of the world. Art is just one way of questioning. The arts seem to me to be charged with making strange what we see and experience so that we interrogate it more carefully. No easy answers in this world.  I still assert we need to develop our capacity for difference and ambiguity. Why do so many people seem compelled to colourless, settled existence? What’s wrong with turning things upside-down and living within our discomfort zone?
The couple you mention exploring psychoactive drugs have actually contributed much to the pharmaceutical world.
You also wrote: “But do we really need our thinking challenged to the extreme?“ Who says that what we are talking about is the extreme? I’m sure there is much more as yet unexplored.Â
You ask the question “Where does it stop?†– and I throw that back at you. Just where do YOU think it should stop, what leads you to make that decision, why there particularly, what will it mean if it does or doesn’t stop where you suggest? By what criteria is your label of “ridiculous†applied in your opening paragraph? As an artist that is the world I live in. I’m not always at ease with the ideas that I consider, and I think that’s the point? Many people in history were deemed “foolishâ€, “ridiculous†and “dangerous†– Giordano Bruno, Christopher Columbus, Nicolai Tesla… should we be without their contributions because we dared not allow them? In Giordano Bruno’s case he was killed for believing the majority was wrong… Edmond Halley was acclaimed some years later for making the same claim!
I’m curious about the subcultures that will emerge in relation to the work of Kevin Warwick (see his book I, Cyborg) – as he was doing his experiment (under strict medical and academic ethics limits) he was getting correspondence from people doing the same work in their backyard surgeries… I suspect the cyborgs are already walking among us and we live in blissful ignorance… how many kids in your class might be experimenting? How many of your colleagues have already made the decision and undergone cybernetic implantation – Can you look at any of us again and not wonder? When you look in the mirror one day will it be you looking back?  Hope you sleep well!!
The future of technology, info systems, computing will very likely become inextricably interwoven with biology, genetics and psychology… it already is… we live in that world – hiding from it and saying it frightens us doesn’t seem a particularly effective (or interesting) response.
No hyperbole in that lot! Of course I might have found an “artistic†way of redefining “truthâ€â€¦.