there is certainly a duty of care owed by tuners when you leave your car with them - they can't just charge you money to tinker with your car and say, oh well if it lets go it's not our fault.
yes they are making the car more powerful, making it do things it wasn't built to do etc, but that shouldn't excuse them of liability - what about LPG conversion places, they use dynos to tune cars - do they not have a duty of care to their customers either?
OP, i think it's best if you discuss these issues with your chosen tuner before any work is done, just so he knows you're not willing to lay down and cop it if he pops your motor.