Numbers on the batteries:
The Voltage is the average voltage during discharge. The higher the better usually. Good packs will be between 1.170-1.180.
Runtime is the run time of the cell from full charge to .9v cutoff during matching. Higher is desirable.
IR is the internal resistance of the cells (the numbers vary based on the software the matcher uses.) Lower is better though. Some cells, i.e. Fukuyama
will have numbers in the teens, IR's of 12-14 are good on those. Some matchers will be in the 3-5 range.
Make sure if you're trying to compare cells, that they were all discharged at 30 amps. Some matchers are matching at 35 amps and it actually will lower the numbers on the label because of the higher discharge rate. Some like Reedy "Realtime" only take the voltage from the first five minutes of the discharge cycle instead of all the way down to the cutoff like most others. Their voltage numbers will look very high compared to others.
Mod motors: Changing the timing will change the power band of the motor. More advanced timing will yield more top end and less timing will yield more bottom end. The lower the turn, the more speed. Single and double winds refer to how many strands of wire are used. 8x1 (eight single) has one strand of wire wrapped around each pole eight times. A 10x2 has two strands of wire wrapped ten times around the poles. Usually a double will have smaller wire than a single. The lower winds, singles, doubles will give more rip out of the corners. Higher will be much more smooth. TC's usually stick with singles and doubles where off road guys like the more mild power of the triples etc. since there's less grip generally.
Brushes and brush springs: That's very dependent on a lot of things. Basically, there are different compounds and spring tensions which give different power capabilities as well as different wear capabilities. Usually you're balancing speed with wear. Different motors like to have different brushes, timing, gearing etc...
Did I just write all that?