It is my understanding that the "A" tires are a slightly softer compound from the "R" tires. When you autocross you get what, 3 x 30 second runs? Not much time to get heat into the tires, so with the softer compound, they will work better quicker.
In road racing you have a tire that takes more heat due to being put through the ringer a little longer, so in order to keep them consistant through a race (even enduros) they create a compound for that.
R'elise me, There are a lot of Elise guys that swear the A tire is the one to use due to the weight of the Elise. They say it comes to temps quicker hand keeps that temp more consistently.
So if I were you, I would mount those suckers and have fun!
Are you racing or just HPDE/Autocross? if the latter, you are fine. Have fun with your gumballs! Even if you are actually racing, I am sure you'll be OK. Your tires MAY not last 8 weekends, only 7, but you will love the Hoosiers.
Have fun.
Dave