there is a reason why there are diff. tire compounds and diff. insert types. Foam tires are also rated by diff. compounds/shores.
For rubber, (e.g Sorex) is rated by temp. range....i.e; Sorex 32R is for when the track is cool with low traction surface, while Sorex 40R is for high. temp tracks with high. traction surface. So, the softer 32R gives a grippier feel and allow some measure of control over slippery (low traction surface) while the 40R is made of harder compound to give it longer life while still allowing for some grip on sticky (high traction surface). Due to the softer compounds of the 32R, tire wear would be higher if used on high traction (sticky), high temp. (hot!) surface. On the other hand, the 40R would be very slippery on low traction surface and the cool temp. would hardly expand the surface of the tire giving little or no traction at all.
Now, the reason we use inserts (instead of a hollow tire) is to simulate tire pressure. Since it wouldn't make sense to miniaturise tubing and pressure valves, RC tires use inserts instead, to allow the tire to maintain a consistent shape in changing conditions.
So, you know Sorex32R gives a grippier ride, but you wish it would last longer on hot and sticky tracks, this is where inserts come in. The hardest inserts simulate high tire pressure and maintains the soft 32R tire shape to even out the tire wear hence longer lasting tires! However, there is a trade-off. By giving the 32R hard inserts, you're actually taking away some of its grippiness...i.e; when you corner, the tires spin at an apex. If the hard inserts maintain the 32Rs tire shape, then only a fraction of the tire surface would be able to provide traction. The reverse is also true, if you use soft inserts, the tires will warp and match the apex to maximise the surface area of the tire that can provide traction.
So, you can play around with diff. tire and insert combinations to find the best to suit the track where you play.
For max. traction, fastest tire wear, use Sorex 32R with softest inserts. You'll be able to hold the racing line better, but for how long?
For longest tire life but minimal traction, use Sorex 40R with the hardest compounds. You'll save money if you're only bashing or just playing around.
Try out the combinations.
Lets say the track has 70% corners and 30% straight and track temp. is ideal for the 40R. For better line holding but reasonable tire wear, I'd go with 40R(soft/med) front and 36R(med/hard) rear. The rear tire will maintain traction longer but could lose some of its grippiness due to the hard inserts (try med!). The front might have some oversteer if you go with the soft inserts but it might start to lose some traction once it wears out a little. You can use the hard inserts, but there might be understeer, so try the medium....or you can use 40R(hard) front and 40R(soft) rear .......and so on.... you get the idea.
I usually setup my cars with a little more traction in the rear since I use a one way up front and I need the braking power. There are other factors that will effect how your car handles e.g; droop, toe-in/out/camber, shock settings, shock fluid, stabilizer bar etc.
Rule of thumb; change one (1) thing at a time and test out the settings. You'll soon learn what works for you and what doesn't.
Ok, have fun!