I think a 360 Slim would fail to drive software sales. I see the majority of XSlim sales being replacement consoles for older Fat 360s. The difference between a 360 Slim and the PS3 Slim, is that the original 360s (most likely) won't have any advantage over the Slim. Whereas the Phat PS3s had various advantages, from backwards compatibility, more USBs and media card readers.
So where I would say more than a good portion of PS3 Slim buyers were buying their first PS3, I believe a good portion of 360 Slim buyers would be buying replacement consoles. And replacement consoles do not increase software sales. So while you might see an increase in hardware sales, I think the increase in software sales would be disproportional.
The numbers on the home page might paint a pretty picture for the 360, it would be just like the RROD, where you dont really know how many are new buyers and how many are just replacing.