Assuming every other move was the same (except maybe not trading for Toskala, for obvious reasons), I'd expect the first 3-4 years of his stint would be great, since the Leafs still had a pretty good team outside of goal during the late 00s, but he'd be like Cam Ward in Carolina by 2011 or so, dealing with a really bad team in front of him and not being able to carry them as far.
Of course, if the Leafs don't trade Rask, a lot of other moves potentially change. Does Sundin leave in 2008? Do we trade Alex Steen, Nik Antropov and Bryan McCabe in 08-09? Ponikarovskya in 09-10? Do we even bring in Brian Burke, or does JFJr stay longer? If not, then the Phaneuf, Kessel, Kaberle and Gardiner trades all potentially go away. Does Nonis ever even get a look to any extent and sign those ridiculous contracts over his short time here? Maybe Rask leads a youth movement in the late 00s instead and the Leafs start to look like they do today in 2010-11 and might have won a Cup or two. It's impossible to know, really.