Definitely stick with UR, as robort said KTK and FRF will be phasing out and playing 3 colors without fetchlands and only 21 lands will be way too clunky.
If you already own a playset of Jace's then I'm jealous. If not, then I would suggest only running 2 as the cost/benefit ratio goes way up for 3+ in a deck like this.
Also, I would suggest running 3
roast MB rather than 2, with 1 in SB. I find mid-range decks to be relatively tough match-ups for prowess decks (and
roast is good against them) and control decks to be relatively easy match-ups for prowess (where all the dragons make
roast suck).
All in all, this deck looks like it'd be way fast and definitely have some solid
staying power.