Tony- are those 15" tires you speak of in the 26-28" height range, or are they like a 245/50, which is 24.6" tall?
Check out the M/T sizes. These start at 24" tall and go up from there. They are using the same construction, tread design, and tire compounds from 15" to 20" diameters. They designed these with stiff sidewalls to go after the pro-touring market and the larger sizes have been used on several high end build ups recently. These are readily available from Summit, Jegs, or whomever and could easily become the de-facto street handling tire as more manufacturers scale back 15" tire production.
Want to get really glued down? Look at tires for Cobra kit cars. They are large, wide, and sticky in d.o.t. and race compounds. http://www.rogerkrausracing.com/Cobra/
