Proper braking technique braking is probably the easiest thing for most people to improve upon and make a large difference in their driving and lap times. The first area most people work on is  proper application speed and amplitude followed by learning to trail off the brakes and carry more speed into corners. 

One spot to pay attention to is the consistency of how the brake pressure is bled off and how smooth this release of brake pressure is. This first graph shows a good release and a release that could use some work. It's important to work on the skill of keeping consistent and smooth brake pressure throughout the release, especially when blipping the throttle in heal and toe downshifts.

Does it make sense to put a $400 AiM SOLO in a $500 Lemons race car? Of course it does! Within our small team of 5 folks, we were each able to improve our individual lap times by comparing our speed, longitudinal G, and lateral G force graphs. Throw in the ability to use the GPS features to compare our driving line and we had one teammate lose over 3 seconds on a one minute 31 second lap. That is a serious improvement!