1) Whats the part that "wears" or "breaks" in brushless motors?
The fastest way to ruin a bl motor is by overheating it. Heat cycles will reduce a magnet's strenght, which is inevitable but can be prolonged by keeping the motor as close to ambient temp as possible. The stator can also be shorted out if the motor is exposed to extreme internal heat which can melt the enamel around the wires. This would be an extreme worst case scenario with bad gearing/timing/load. Also, the rotors have an RPM limit, which if surpassed, can shatter the magnet due to vibration/centrifugal forces (I've had this happen with a non-sintered rotor).
2) What can/should you upgrade in brushless motors? Rotors? Bigger = better?
Standard bearings can be upgraded with ceramic, like all other bearings in the RC world, but can be much more important in a motor due to the much higher RPMs they are exposed to. Some bl motor manufacturers offer different size rotors for different applications. More of a fine tuning thing, but a bigger rotor yields more torque by increasing the diameter of the rotor ex:12mm vs 12.3mm (think of it as torquing down a bolt with a 11" ratchet vs a 16" ratchet) and by getting the magnetic fields of both the stator and rotor closer. By moving to a bigger rotor, keep in mind that the overall RPMs of the motor will also drop (ie the kv will be lower).
I don't necessarily see the need to change rotors, as it has a very similar effect as changing gearing to suit driving style/preference. I would sooner upgrade bearings than change rotors, especially on a mod motor. For a 13.5, I wouldn't worry about it unless the supplied ones fail.
3) What is the relationship between temperature and gear? I have heard the both under gearing and over gearing will raise temps?
Undergearing overheats the motor (whether it be brushed or brushless) by having it unload unnecessary RPMs, similar to trying to keep your daily car in low gear on the highway. Overgearing overheats the motor by creating a heavier load on the motor than what it can handle. I've found brushless motors to be quite more forgiving than the brushed motors I used to run, however, I've also seen a brushless motor cooked in both situations.
4) Timing. I have seen both timing done on the motor or programmed in the esc.
Most motor/esc combos offer timing adjustment to be made in one way or the other, but not both. ESC timing is regarded as a more sophisticated method than endbell timing, and is rightfully so when an ESC applies it in a variation system. Much like variable valve timing on gasoline engines have made it possible to have both low end torque + efficiency on low RPMs, and more horsepower on higher RPMs, some ESCs can give you an edge in performance. Such ESCs include the MMP with the CHEAT mode, and the Tekin RS Pro. I believe this method requires sensored motors; to be able to determine the RPM of the motor in which to kick-in the timing.
5) What should i look for in a 13.5 motor for racing in stock/super stock?
First thing I can say is that not all 13.5 motors will perform the same. Not all 13.5s have the same kv rating or torque output, despite the use of wind labels to categorize such ratings. I would first find out what other racers are using at the track you want to race on, and make a decision based on that.
6) It seems like almost everyone wants to use the Tekin RS Pro. Why is that?
(See above/#4), and as stated, because of the quality of the FETs, and also customer service. I'm not biased, and believe both the MMP and RS Pro are great ESCs from great companies.
7) Why would you ever want sensorless over sensored?
Sensorless is a lower priced route, due to the simpler manufacturing process, but is subject to low rpm 'cogging'. Sensored provides a smoother powerband, no 'cogging', but is subject to more parts failure. My rule of thumb = Sensorless for 'bashing' and budget racing, sensored for more serious racing (if you get serious with racing, get serious with the extra coin).