It was an obvious immediate improvement over a stock, without really tuning for speed and just throwing on a 26 tooth pinion I hit 31 mph with a cheap 4200mAh 7.2v stick pack in my Rustler, probably could have been better with more tuning.
I have an armature that I pulled out of a Titan 12 turn and you can tell just by looking at it that they really don't use much copper when making it when compared to other 540 armatures. The Titan armatures has lots of air space, it looks like there is enough space you could make a second set of 12T over the existing ones if you wanted which isn't the case with the 540 armatures I've seen.
Just keep in mind that the XL-5 is only rated for motors as low as 15 turns.
If you can afford it, steeping up to a brushless system is a huge improvement over what the XL-5 will handle and will also be much easier to maintain.