That makes for about 470 k per km captured, and they're not claiming 100% efficiency. So it passes that sniff test, at least. Weight is also plausible.

Final q (other than cost): how much exhaust backpressure?

This is a crucial question! To our surprise, our beta testing of our prototype has actually shown that this is creating < 0.5 psi back pressure on the engine (negligible) and we believe we can get it down to essentially 0 by the time we deliver our first units to customers this summer.