I've been experimenting with a Bplus puck screen with my Cremina SL. Having great results but wondering if you should heat/warm the screen before using it? Or is the thermal mass of the screen insignificant in impacting temperature of the shot?

I don't think it is insignificant. Some are quite thick. Not sure how thick yours is but a preheat when you preheat portafilter and basket with a flush should do it. Work quick after getting coffee into basket and preinfuse for 10 seconds.

Nice machine.

Bplus says the screen at room temperature will lower the brewing temp by about 2-3C and recommends upping your boiler temp to offset the loss. While it's easy to preheat it to lessen the effect, it also cools down pretty fast due to large surface area, something to keep in mind.

When I first got mine, I tried holding them to the steam wand with tweezers before use. After a few goes, I tried them without heating. I never noticed any difference. Admittedly, my machine isn't a lever, but I don't see how that would make much difference.

I wonder, however, if the bother of using puck screens is worth it. I definitely get drier, denser pucks with them, and they stop the shower screen from being fouled with spent grounds. On the other hand, I don't detect any difference in the cup. Some say they reduce or eliminate channeling; to test this, I intentionally badly tamped a few shots. I had channeling. I doubt I'll keep using them, but it was fun to try.

They make a certain amount of sense for the Cremina SL as it's darn near impossible to remove the dispersion screen without tearing the gasket. Saving the gasket is possible but you need to disassemble the group. The puck screen keeps everything nice and clean so you don't have to clean as often.

Agreed! It's been nice to use one with my SL (and I wish I'd had one with my CT-1!)

FWIW, usually leave it in the empty PF basket that is on the group, so it's pretty warm when I pull shots.

That's genius! I'll give it a "shot".