I'm not against the idea per se, but honestly it's quite easy for a given player to know the rates and multiply the differences on their own. The info is out there and it takes no time at all.

I mean if the devs could knock this update out in 5 minutes I'd be all for it; but this would take a tremendous amount of time which would be better spent elsewhere.

Very considerate of you to look at it that way, I agree it probably would be a lot of work and I'm not sure of the overall benefits of it. I do think that maybe the bespoke slayer NPC's could benefit from having XP rates listed, such as Golems, Burrowers etc.

The time the developers have got are best used elsewhere at the moment with the likes of a slayer update in the works and other things that players will be able to enjoy. Unfortunately there's a very long list already of things that need fixing/adding, but these are insurmountable in comparison with the 'major' updates that are being worked on right now.