Actually I don't think the cap is supposed to raise before you pass 1500. And yes, arena rating does influence the amount of conquest points you get. I know it says that on the conquest tab, I checked it just last night.
The Conquest tab specifically refers to your battleground rating, and the neat little tooltip only appears on the battleground rating conquest tab. This is why there is some confusion. Why would it be on the battleground tab and not on the arena tab? Obviously space is a concern, but bliz still could have been more clear.
There is no battleground-specific tab, the conquest tab is for both arena and rated battlegrounds. Really though, why wouldn't arena rating increase your conquest cap? Blizz won't just suddenly make arena completely irrelevant.