I run an SSM AutoX Miata with a ISC Front Bar with 0.188 wall thickness. I'll be running 275 Hoosiers on 15x10s and 800-850# front springs on valved double adjustable bilsteins from Fat Cat Motorsports.
It is a much stiffer bar than the mere 1/8" size increase suggests. Since it has Aluminum Splined Arms it doesn't bend/give in as many places as a formed bar will, like the racing beat. In my opinion, you are going to be better spending money on shocks and the proper springs. If you're tracking a lot I would recommend 700#F/400#R, with a good set of shocks, that is more streetable than you might think.
Put your money in SHOCKS though, I'd highly suggest calling Shaikh at Fat Cat Motorsports
and getting a full coilover kit from him. He can advise you Shocks, Sways, Springs and Valving to compliment his kit. Even if you go with the base revalve coilover kit, it will be worlds better than any off the shelf coil over setup. While you're likely to spend an extra +$500 on custom valved setup over OTS shocks, trust me, you will be happy that you did it right the first time.
At least give him a call to discuss your thoughts before you buy anything, he's very helpful. Then after you've done those things, if you're still unhappy with the body roll, then upgrade to the ISC Sway Bar, but I doubt you will need to, your current RB bar is plenty with the right setup.