Well for starters it depends on the info Obi has on Itachi. If we asume he knows bout the Uchicha and what they ken do ( genjutsu) he would simply close his eyes. Remember ghow Luke trained to deflect the shots while heaing a helmet on, or for that matter the aprenticens in ep 2 with yoda. The ca sense the enemy with the help of the force, so in theory he wont get hit by sharingan. Now for the speed factor, imo equal, itachi hs a great casting speed for the seals, so he can spam some powerfull jutsus, on the othr hand, the jedi can deflect a frcking blaster shot ( matrix style ftw ). So imo boyh are equall at this.
In the end it would come down to the weapons used, a jedi lightsaber can cut a kunai, unless its mandalorian steel ( like the armbands used in some comics that coused the sword to lose power

) So unless Itachi has kunais made from mandalorian steel ( or whatever that material was) he is ****ed ^^