I guess you could use a browser cookie in addition to the other security measurements. Still no effective protection, but might decrease the amount of cheating people.
doubling the hashspeed would double the expected value for generated bitcoins in a certain time. as long as: expected value - tax = higher than the expected value before doubling the hashspeed (without taxes), I guess I'd use it.
this needs to be investigated for the sake of science, i promise i'll post my results honestly, i just sent .03 bitcoins, i'll edit this post as the bitcoins start rolling in