Here are some of my observations after many years of racing the M18 week after week on our very tight medium traction track at "realistic" speeds. I've also driven my setup on RCP (high traction) and it worked fine there also. Almost all the cars in our little racing league have the same set up.
Although it sucks to admit that "plastic" is better than fancy shmancy carbon fiber, after testing I could not deny that the composite (plastic) chassis makes the car have much smoother and more predictable behavior. So the standard is definitely fine, unless you just want the cool factor of the pro.
Do something with the spring/shock assemblies. Get the shorter springs with the preload spacers at the least. I also came up with a totally different spring system that gets all the friction out of the suspension and steering and makes the handling even smoother. This is briefly mentioned and shown on my website on this page:
http://www.harnoishobby.com/cars.html.
Get the 6-degree caster front upper A-arms and the Adjustable turnbuckles. The standard has 4 degrees fixed rear toe-in and 12 degree caster, which definitely caused weird handling issues such as "wondering." But I have seen it work well out-of-the box on high traction surfaces.
And you'll almost surely want the servo saver. Read the xray product description to check, but I think this comes with the RTR but not the standard kit.
For slicker/smoother surfaces like basement/garage floor, kitchen floor, gymnasium, etc, the combination of hard front and soft rear foam tires really helps stabilize the car, and will last a long time too.