If I can add my $.02 here with what I have found. I am not an SC8 expert by far, but I have been racing the crap out of mine. There are a couple of points that seem to be on the table I would like to address. These are only my opinions so take them for what you paid for them.
Tires: I feel a realistic scale tire is the key to the CORR trucks. I wouldn't race my truck if any type of buggy tire was allowed in a SC class. Many of you are talking about the Moab tires, which look to be a good tread and realistic look however, I don't feel they are the best; although full disclosure, I have not personally run Moab tires. Pro-Line Switch tires kick ass on these trucks. They have the exact look you need, they fit the SC8 wheels and they are the perfect combination of traction and drifting. I'm not saying they are the best out there but I couldn't be happier with the way my truck performs.
Engines: I'm not going to touch on the point of what is a better performer a .28 or a .21 but I will throw this point out for consideration-again taken from my experience. These trucks will fly with a 21 in them no doubt, but that is not what some (or most?) of us are after. After all, these are Short Course trucks meaning that the tracks they are designed to run on are not necessarily going to provide long straightaways with chances to open up a 21. Now if you're racing on an 1/8 scale track, it may be a different story. Although I don't race consistently on an 1/8 scale track, I have goofed around on one and still feel the truck can hold it's own. The track I race on is a converted 10th scale which really makes the "short course" aspect of the truck shine. Torque is the key here with quick acceleration being-I believe-a higher priority than top end speed. My times slowed by taking out the stock 28 motor and putting in a 21. I put the 28 back in. I think this was due to the 21 not being able to build the torque as quickly along with not being able to take advantage of the 21's high RPM's. Remember that the tires are a limiting factor and no matter whether you have a bad-ass OS Worlds, or some cheap $99 hobby motor, if you can't get power to the ground, it's useless.
Personally, my next motor upgrade will either be a 28 or even a 30. I am even considering a cheap Traxxas 3.3 for shits and giggles just to see what it would do. (there are a couple of Slayers around that absolutely fly). I will gear it for top end and let the stronger torque drive the higher gears around the track. This would be the best of both worlds in my opinion.
YMMV