I've only tried the VTA tires on my old GT car (AE TC3) once last spring. I wasn't real fond of them and felt that ultimate grip was better with the belted X-patterns. This was on medium-grip asphalt, but I will have to say that I did not use any Paragon or Simple Green for that matter.
As for the 26 mm X-patterns vs. the 24 mm Advans, I haven't had a chance to compare them first hand. I know one of our local GT leaders (Aaron) swears by the Advans.
BTW, during the EOS series Aaron was neck and neck with Randy at M&M (high bite asphalt, lots of sugar on the track). Randy ran the VTA tires on his GT car at that location and Aaron ran the Advans. I was running belted 26 mm X-patterns. All of us were doping with black can Paragon. All three of us held things pretty tight until I choked in the main. Not sure what tires Randy ran at the other locations in the series, but it's a good bet he was competitive with whatever rubber he was running on.