Yeah the coffee sitting in the hot portafilter basket is going to care about the warm tamper

/sarcasm

That sounds a bit unnecessary to me. I can't imagine that the amount of heat or cold conducted from the tamper to the grounds would be enough to "shock" the grounds and affect their flavor. Maybe if you kept your tamper in the freezer, it would be cold enough to make a difference, but even then I doubt it would change the flavor in the cup.

.... not to shock the coffee? Seems like an odd thought to me, given the miniscule amount of contact a tamper makes with coffee, even when heavily compressing it. Heat transfer is primarily a product of time and thermal conductivity. The time is minimal, and coffee is most certainly not a conductor (indeed, with the amount of air in a dry puck, it's quite the insulator). In any case, I believe that coffee exits the grinder chute at usually no more than 80-90F, maybe somewhat more than this in an extremely busy cafe. Assuming it's around 70F at room temperature, I'm confused why you would ever thing that a tamper at that temperature would affect the puck more than one that's been sitting on the cup warmer. I insulated my Elektra's boiler, and it's still around 135-140F on the top of the cup warmer (checked with a Fluke ) I wouldn't be surprised if a Synesso/LM with a steam boiler rocking 1.5 bar, not insulated, is even hotter than that up top. I don't know why one would expect such a hot temperature piston would be less shocking to the coffee than a room temperature one, not to mention 2-5 seconds after you tamp, when the water is hit with ~200F water at 9 bars of pressure.

Anyway, that's entirely more thought than the issue likely deserves. This is most certainly a superstition.

Which is not to offend or berate your barista friend, Eric. I am sure he makes a fine cup of coffee. Attention to detail is a plus, but even the best of us get caught up doing strange, ultimately pointless things with the idea that they improve results in the cup.

Get two tampers, one prewarmed, another cold. Ask your barista friend to prep two portafilters and hand them to an assistant who locks them into the groups without saying which is which. Now ask said barista friend to identify the espresso pulled using the prewarmed/cold tamper.

I'm not being facetious; a blind taste test quickly settles the difference between useful tips and odd superstition.

My money is on odd superstition.

I've been digging my lead tamper with asbestos handle, actually.

OK, so I realize that a little coffee grinds falling through the ventilation holes on the cup warmer can make the machine dirty, but 'havoc?' Really? That seems a tad dramatic to me.

Exactly! Imagine how "shocked" these already traumatized grounds will be by that sudden chill!