Whilst the Heirloom's can be changed at anytime, I actually do think that's exactly what they will stay as, 90-10.
Speed of levelling, same with any expansion, it's always slower at first, then they nerf the exp rates later on, if you level in MoP now, it's faster than it was when it originally came out, same happened with the others.
So my vote was 10 levels same speed.