Good day HB
so I was just pondering.... is there actual physical limitation on thermojet for espresso making? Or is it just Breville cutting corner to make their thermojet machine less than ideal?

Cause I think it has great potential, and the main problem shown in bambino machine is the thermal stability, creating unconsistent shot.
If instead of using the breville controller, we build an arduino controller to have better control on the flow to match the heating capability of the coil, wouldnt it allow for consistent temperature shot while having the super fast heat up time of the thermojet? I see this good for home use as most of the time we are just making one or two shots at a time, and we dont need to waste the energy to heat up a large thermal mass just having it cool down after 2 shots. To keep things simple, group/PF heating would still best be done by doing a shot (may be programming a superheated shot to speed that up?)

Does that make sense or am I completely out to lunch?

Thermojet/thermoblock should be able to handle this task. Companies such as Ascaso are betting on it.

That said, perfect thermal stability might or might not be the ideal thing for an espresso shot. Consider the following: if you have a shot at 9 bar throughout, and at a perfect 200 degrees throughout, then by the time 25 seconds have passed you've extracted most of the good stuff and are now pulling mostly bitterness. And pulling more of it, because as you extract solids the flow will increase given the same pressure.

If the temperature dropped though, the bitterness you're extracting toward the end of the shot might be ameliorated a bit.

I'm not saying this holds for all coffees. Some might indeed benefit from perfect heat stability!

Just something to consider. As for Breville, I doubt their motivation is to make their machines less than ideal. I just think they understand their market, and are doing what's needed to maximize profit in that market.