The longer motor plates do allow you to use a larger spur gear, but they also lengthen the car's wheelbase. This affects the cars handling, making the car feel a little more stable but at the cost of steering. The lowered plates don't affect handling other than what you get when you use a smaller diameter tire. As you can see on the IRS website, the motor plates come lowered only or lowered and lengthened.

I used Niftech lowered and lengthened plates on my Quad 12 when I ran it on asphalt, and it worked quite well.