I've never worked with elm but everything that I've built with oak (red oak, furniture grade) has been heavy as a mutha. Stained and sealed, it looks nice but it weighs quite a bit more than "normal" tonewoods and, as others have said, might be back-breaking to stand with slung over your shoulder.
If it's just a topper then it may not be so bad; I'd be curious to hear about the results - if there is any noticeable difference in tone, etc, if you end up going through with it. Hell, I might build a body out of poplar with an oak top just to see. Once I get done (get started) with my other build, that is.