Tri-City Paints (TCP) is a local company here in San Diego and this product is one of their house brands. I personally have not used this particular product but I don't see that it would be much different than any other 2-stage Urethane. I have used the Kirker brand which they call Hotrod Black (also comes in other colors but most people use black). This product is made to be shot without thinning but you can thin it somewhat to suit your taste and technique when spraying. Slow reducers are only used in very hot temps and fast reducers are used in only cooler temps, everything else is medium. It has been my experience that the slower the reducer the higher the price. You will have to try a sample spray of this product to determine how it works in MS...temp and humidity have a lot to do with how well it sprays and flows. Cure time should not be significantly impacted by the small amount (none) of reducer that they recommend...even if you choose to add reducer the cure time should not change. My recommendation would be to save your money and not buy the reducer with the kit...most any urethane reducer will work just fine, buy it locally and save the shipping.
Not to hijack the thread but I know of a great inexpensive alternative to this paint system. You can go to your local John Deere tractor dealer and pick up a gallon of Blitz Black tractor paint. Grab a gallon of the reducer and primer as well. Total investment is less tha $75 last time I checked. I sprayed my Plymouth Scamp in the driveway and it came out great! I am by no means a painter but this was childs play. I used a cheap HVLP gun but you can also get this stuff in rattle cans.
Be careful of runs and dips, it sprays really thin. Can end up looking striped or patchy. I had my '59 painted by a local guy using hot rod flatz. We went with a satin black mix and it turned out awesome.
I did a little Hot Rod Flatz research too. This is a single stage paint....so mix it, shoot it, let it dry and it won't need any further clear coats to seal it. CharlieLed is correct on the reducers being to account for the temperature and humidity of the day. If you live in a hot humid climate you'll need to slow down the reducer or you can get a thing they call "solvent pop" where the reducer bubbles up under the paint as it drys. Otherwise, I've always been partial to single stage paints for the do-it-yourself projects because I have fewer items to buy. Some of the two stages systems need seperate reducer and hardener for the clear coat, so it ends up costing quite a bit more.