Is the power increase even that much? Honda is terrible at publishing power numbers (so are the others, but in the opposite direction). Is some 30cc worth all of 1.3hp? Does that offset the weight difference? I was looking up stats, but I've already forgotten. I can't imagine that the 520 isn't but quite a bit heavier given the aparatus required for the dump bed.
As for having a dump bed: I never understood why this was such a big deal for small SxS users. Who cares? The rack is so versatile, you can put anything you can think to make on it... including a dump bed if you figured out how to mount it. The aftermarket is making some really silly stuff these days, stuff you couldn't mount on a 520 without first taking off the dump bed.
Where I live, trail restrictions are based on weight. Many are already off limits to the P-500 based on gross weight, but if we keep on with "feature creep" as the P-520 does, it will soon be too heavy for those trails limited by "dry" or "curb" weight limitations which the P-500 currently squeaks past.
IMO, when the P-500 came out, when I was selling them or trying to, and even more now, Honda needed to have an under-$10,000 out-the-door SxS option. For about five minutes in 2015, that was the P-500. $10k total cost before accessories is a magic number. I think a lot of people who don't think they can afford a SxS would be happy to buy a brand with a quality reputation at that price point. Instead, Coleman and others are selling really poorly made knockoffs assembled in China, for only a shave less than what is offered by Honda. If I were to spend several thousand dollars, and all I got was the basic 50" Cub Cadet, I might just swear off SxS' forever. (BTW, pretty sure "cub" and "cadet" are ripoff branding from Honda).