Frames like the Mash/Cinelli and the Affinity Lo-Pro aren't necessarily pursuit bikes, rather just bikes with larger triangles. I've test ridden a Lo-Pro, and I couldn't actually feel the sloping top tube (as I generally have a few inches of saddle to bar drop anyway), as long as it's set up properly anyway.
That being said, it was explained to me that the larger triangles on this sort of bike make for stiffer joints, which allows the tubing to be made thinner without compromising the structural integrity of the bike (and making the bike lighter at the same time). I definitely noticed that the affinity frame was lighter than my current bike (Kilo), but that's not too impressive I suppose.
I've seen a lot of the Cinelli/Mash frames here in San Francisco, especially at pretty much any race (I think the Rainier has one that Jason Yim was racing for a while)/