Pitch is the angle of each tooth (also related to the size of the teeth themselves) on a given gear like a pinion or spur gear. As pitch sizes go, 32 is the coarsest, meaning the teeth are large, and engagement more positive. These are usually used for 1/8th scale and off-road. 48 pitch is the overall standard for nearly all RC manufacturers. Nearly every RTR you're likely to buy will come with 48 pitch gears. 64 pitch is the finest, with the smallest teeth, and is normally used for on-road, and are usually found on indoor racing chassis.

Your spur choice is ideal because it lowers the overall power output. That 13.5T motor is overpowered for drift, regardless of what you've been told. It's not majorly overpowered, which is why I say you can compensate for it, by lowering your FDR. This would not even be an issue, if we were talking about 50/50 drift, but we're not. CS has limitations, and the smoothest power output is the most important. Unlike 50/50 drift, CS steering inputs are, by comparison, smaller and more frequent. With an overpowered power system, you are not just trying to steer smoothly, but the power output is fighting you, every step of the way. What you are trying to do, is match power output to steering, so both work in unison, rather than have two systems that are at odds with each other. Take what I said about overdrive "pushing" against the front wheels....That's a part of what I'm describing. You are already working with that against you. That's where smooth power delivery is important, and slower systems provide that smoothness. The 17.5T BL system has been found to be the ideal for drift, and is also the standard for most stock racing classes, too.

My suspicion is that you were pushed to buy that 13.5T system, because 13.5T is going the way of the dinosaur. Racing classes that use 13.5T are slowly disappearing, and most shops want to get rid of their stock of that rating any way they can. In a sense, you can take what you were told, about that 13.5T system, is just a part of that.

O.K., when I say "take up the upper half of adjustment range," do this:

Draw a line down a piece of paper. The left half will be labeled "13.5T." The right half will be labeled "17.5T" Now draw a line down the middle of the right half. There should be two lines on the paper now. Now fold the paper in half, so all that shows is the right half. Now....the paper to the left of the line is the upper half of your adjustment range (as regards spur/pinion choice). That's what that 13.5T system takes up, in adjustment capability. There is an upper and lower adjustment range with every motor size. What you have done, with the 13.5T system, is take up the upper half (designated by the left half of that 17.5T labeled side) of the 17.5T adjustment range.
You are now left with fewer choices, to adjust your power output, follow? Each motor has an adjustment range. When motor choice is not ideal, whichever end of the power spectrum the choice is (in your case, it was 13.5T, or the upper half of 17.5T, the lower half would be a 21.5T motor) takes away from your available choices. In other words, the right half of the paper now doesn't apply to you, any longer. This is the adjustment range that is lost, with the 13.5T motor.

If you are running a 22T pinion as stock, just get the spur. Try that first.