As far as there being a benefit, yes, there's always a benefit to using an external recorder. I use a Canon 7D, unfortunately I don't have manual control over Audio Levels nor can I shut off the AGC like the 5Dm2 can. You get cleaner audio from using an external recorder, you can monitor it, better controls, etc.
As far as the H2 and the 5Dm2 paired, I can't say. I do personally use the H2 with my 7D and it works great. The H2 isn't the ideal recorder though...an H4N would be better, XLR inputs...the H2 was suitable for me because I moved from an Sony VX2100 which all mic inputs were mini...that's mostly what I have, so the H2 is great for those.
If you've got the cash and haven't bought the H2 yet, maybe save for an H4N. The H2 is a solution that I use, it works, syncing is easy (just make sure to slate or clap before recording) and it does work great, but for that perfect audio solution I'd get an alternative with XLR.