For first timer AutoX just go with whatever is on the car. First several events actually. You really want to be more worried about not getting lost than you do going fast.
Cause, trust me, your gonna get lost in the sea of cones the first time. And when you're not getting lost, you're going to be over driving it. Then your timing is gonna be all wrong. This is pretty much going to be your first 2-3 events. By this point you'll be either done with it and never going back, or you'll be looking at all kinds of things you can improve and tires may or may not be at the top of the list.
Regardless, depending on your SCCA Region location and the course they run you likely won't be seeing speeds over 40-50 mph or anything beyond 2nd gear. And again, navigation is more of an issue than traction will be.
Do look up your local chapter and go through their "Your First Event" page- this is the one for my chapter- Dixie Region SCCA - Your First Event
Again, in case I haven't said it enough, do a few, then see if you want to invest $$. Lotsa folks find they like the driving part, but not travel, the working and other stuff that goes with an event. You'll spend the day there, rack up maybe 3-5 minutes actual drive time, and spend literally hours chasing cones while other are driving. It's a blast, but not everybody's cup of tea...