Interesting that the assumption in the op is that the 360 HAS over-saturated the shooter market so Gears 2 wont have a big system sales impact but the PS3 HAS NOT and so KZ2 and Resistance 2 will be system sellers.
I think that is a very convenient argument that is well IMO, flawed. I'm just not sure where you got your statistics that show that every shooter fan who wants a 360 has one, where as every shooter fan who wants a PS3, does not have a PS3.
I'd venture to guess you don't have statistics that prove that though, and that this is just your feeling of the market. I can understand that though, as sales prognostication, particularly on forums is really a "feel" art, however I respectfully disagree with that assumption and therefore do posit that Gears will move systems. To what extent, I'm unsure, but do expect there be a hardware spike when Gears 2 is released, particularly if there is hardware price reduction in place at that time.