Lots of guys have upgraded to the xtra-10, there is a ton of info on it online in various places. It's not a simple "bolt in and go" installation, the mounting hole locations are different so you have to remove the mounts on the tunnel and redrill holes to accomodate the newer suspension. Getting the right dimensions and measuring twice, drilling once seems to be the key.
As far as interchangable parts, the carbs are the same carb but with different jetting (both use Mikuni round slide 38mm carbs). Some of the other parts such as primary and secondary clutches will be the same, but with different springs, ramps, weights. I"m not certain on the engine; the case may be similar, but the pistons/cylinders will obviously be a lot bigger on the 800 vs. the 600. Pipes and exhaust will be different and the 600 pipe will not work on the 800. Some of the 800's had triple pipes on 'em and were OMG fast, (not for the faint of heart!) The triples need more attention to jetting changes for temp and elevation over the single.
Good luck with your decision!