I am just curious if it is better to use a firm insert instead of a soft or medium to help keep the tire from changing shape under acceleration and cornering?
It all depends on how much traction the track surface has. The more traction the stiffer the insert. The less traction the softer the insert.
I have been using sorex tires with sorex medium b inserts for a few months racing stock and have had good luck.
I feel they are the best tires, wheels, and inserts on the market today.
I have noticed though that after assembling them that they do have a bit of space in between the tire and insert.
The air gap will help with how cool the tire runs and also with the amount of traction the tire will give. More of an air gap will give more traction because the tire will not overheat as fast when you push the car over the limit.
I will be getting some new tires here soon and was going to try a med A insert which is Sorex's thick insert unless it will cause problems?
It shouldn't cause any problems. I run all of the inserts and they all work excellent.
Also, I am fairly new to Sedan, how long do these slicks last racing stock/mod?
These slicks should last you about 1-6 months if not longer. I race every week and I can go on a set of tires for quite a while. It all depends on how you set up the car and what conditions you run the tires in. Also depends on what tire/insert you are running.