I think that giving XP on the post reputation is a bad idea, because the average reputation of a post is going up. When I joined Perlmonks (1 month ago?), the Best Nodes were about 15 to 20 points, now we have 60 points. If I think of the last two weeks, the Best Nodes of the week were also more or less the Best Nodes of all time. So I think there is an inflation in votes, as is to be expected by more voters. I also think there will be an inflation of XP, as long as XP is coupled to the raw votes and not somewhat sensible to the total number of votes.
Of course, another argument might be, if we have more voters, we also have more posters and the votes will be somewhat distributed over these nodes, but I think that the number of new nodes dosen't rise in the same way that the number of new voters rises.