Taking 35 ml out of a 2L boiler on commercial machine, and replacing it with 35ml cool water.....barely changes the temp. , And doesnt affect it at the grouphead at all. The effect on the gaggias tiny 100ml boiler is 20x, requiring high heating to be applied during the shot, necessitating good control.
Since I'm the only coffee drinker in the house, I don't find the small boiler capacity of the Gaggia to be a problem. You'll definitely run into problems with heat recovery if you're looking to pull a bunch of shots back to back. The PID controller does a pretty good job with this.
Ultimately, this was just a fun personal project that I've been able to benefit from. I've also got Home Assistant communicating with the controller now, so that I can remotely turn it on and have it warmed up ahead of time. I've got a 3 hour time limit in the code, which cuts power to the boiler, as well as a 10 minute timer for steam mode, as it's probably not wise to keep the boiler at 125C for too long.