It's really not so much a problem. 30mm tires are nitro/asphalt. For years we had run 28mm tires on carpet. Jaco only recently released the 26mm tires. I ran them, and like the 28mm tires better. There's no reason it shouldn't stay open tire.
Rubber tire races are much harder, there are so many different tires out there, along with differet inserts, different rims... The combinations are limitless. I can see where they can get really expensive trying to track all that stuff (rumor has it Masami has a library of tire inserts he drags to races).
For the nats I took along a couple of sets of tires (same compounds) that I know my car works on, and borrowed a set from someone else to try of a different compound/width. I went back to what I knew worked. The same tires that I use for club was also my tire choice for IIC.
Speed Merchant Rev7, Tekin, TQ Racing (wire), Team Tamale
RC Excitement - Buy where you race, support your local tracks.