oh no doubt that run flat tires are very heavy, but the technology is there, it just needs more financing by enthusiast/sport tire companys. larger tire diameter can help you only if you have the torque to accelerate with larger tires. because basically adding larger tires decreases the gear raito(kinda like going from 4:1 to 3.8:1 or so). because by adding larger tires, your engine isnt rotating as many times to complete one rotation on the tires, thus requiring more torque to accelerate. like i said, those civics that have huge rims/tires, are for show, because they usualy(i mean usualy, there are the true tuners that have 200-300+whp on them) they dont have much more than an fart can and a "cold air intake" which doesnt help them by more than i would guess 10-20hp. and as always i have always liked the quote which i have taken from a sport compact car book "no replacement for displacement".
but wider tires give you the larger contact patch. those are the ones that will increase gripping performance. you can have 15 inch rims with super wide tires, and that would help you more for gripping than having 18's with not much wider patch than stock tires. then again here comes the compound type. having a r compound tire will increase grip considerably over most compounds, and picking the right manufacturer can help even more. but of course you know this(not doubting this, just putting it out for everyone else). nitto, hoosier, dunlop, goodyear ect ect... not all r compound tires are created equal. the biggest thing in most modern cars thats overlooked now, is tires. its a sad fact honestly.