Glad to see the tire debate continuing in earnest here, as we're still going through the same thing, and it's the last hurdle to having a solid, unified class. All of the other aspects of the spec have been spot-on (21.5 spec/GT bodies & wings/etc.). We're at the point where we're getting 20-25 racers, but they're still split into two camps: X-Pattern and Solaris premount.
The pace is about the same. Solaris users enjoy simplicity, grip, longevity, affordability and decent looks at the expense of variety and uber-cool rims. X-Pattern users enjoy 3 tire and infinite insert choices, perfect scale looks, and different offsets to fill out 200mm bodies at the expense of price, labor, and wear.
At this point we're kind of at an impasse. Perhaps a nationally-unified ruling would sway us to one side, although many seem to have a religious fervor for their preference.
Personally, I like the Solaris option, but I'd jump ship if it meant we could all run the same thing and not split up the class.