Is anyone using this screen on a spring lever machine? Just curious if the "back pressure" will be an issue with the falling pressure as the shot is being pulled.

This is just a theory:
Since the screen needs some pressure to let water through it might lead to less dripping after the shot in a lever machine. Since the water above the screen will not complete drain but stop when the pressure is lower then the screens threshold. On the other hand that might lead to additional wear on the seals.

And I think you are right, it could lead to water remaining in the piston which could also affect temperature of the next shot and could lead to additional scaling in the cylinder. This is wild speculation thought. I think the force if the spring and the back pressure need to be determined here.

PPS: I am very much looking forward to getting my hands on a matrix screen and using it on my E61.I think it was designed for pump machines. I expect 2 main benefits. Less channeling due to even water distribution and faster saturation of the whole puck. The second point is very interesting to me as a fast saturation should lead to more even extraction between the top and the bottom of the puck. I am thinking of a similar effect like the one way valve just above the puck of a La Peppina. I read multiple times, that the Peppina creates better cups then most home levers, maybe that effect is the reason.
And in terms of the first point, I believe that the screen could be able to deflect the effect of bad water inlets in the group above the screen. While E61 is not too bad I could imagine a Silvia benefitting from this much more. At least I read that the Silvia struggles with this. Of course Rancilio would need to send a test machine to develop the proper fitting screen

Another possibility on a spring lever might be the dreaded "blow out" when the portafilter is unlocked if any substantial amount of back pressure is held by the flow resistance.
If I were trying this the first few times, I would probably wrap a towel around it, just in case of a mishap.

I dont think the back pressure from behind the screen could escape in that case, except if you are worried that the screen is plopping out.

Exactly! On the Bosco the only thing holding the screen when the portafilter is removed is the gasket friction. It's really not a secure fit until the portafilter is in place.

How high is the back pressure actually?

hey!
backpressure is about 1.5 bar. it's not something you can really see on the pressure gauge, but in machines connected to the mains, we could see a difference in the flow without the pump on between 2 groups, one with matrix and one with a normal shower. the flow was twice as slow with the matrix, the mains were at around 2.1 bar. also, like a gicleur, the pressure drop is only during the build-up of pressure. once the pressure above and under screen is the same (requires a coffee bed), that's it. the flow has to be equal.

during the experiments, we worked also with screens with much higher backpressure. the change on acidity was much more noticeable, but without the portafilter installed, the shower would pop out from the pressure. it would probably happen also with the production model of the matrix when the gaskets are old.

the matrix works also on a lever machine. the pressure profile doesn't go to 0, but wherever the spring is stopped. around 2-3 bar on the machines I've worked on. besides a cleaner group, i don't think the matrix would have much to add. the lever machines in terms of flow are near perfect. you wet the coffee for as long as you need, engage the spring that assures a stable flow. a mesh shower screen would interfere with the flow as little as possible so you'd be basically pushing a cylinder of water through the coffee puck. no turbulence.

so far, feedback from users is generally good. we had one unit that was badly produced and was sent back to us.
we have 2 cafes working with it alongside their normal showers and they've already decided to switch all their groups to the matrix. we have 3 of the best machine makers testing it and they're already interested to add it as an option.
we also started working on a La-Marzocco version.

the matrix has it's advantages and disadvantages. like all screens. we're hoping to carve a part of the market, but it won't we as widely used as the cheaper options...