I like your work. If I ever manage to get myself a decent espresso machine, I'm going to try a PID controller mod on it.
One thing that interests me is whether you could "autotune" your PID controller using a step input (ie. turn the heater full-on), measuring the response (the lag Td and maximum slope of the resulting S-curve) and applying the method as per part D at the end of this.
Regards,
Sam.