Not a ton of improvement in speed, but some. Particularly, better leaving a stop sign, and better up a steep hill.
You will double your range with 12 ah of lithium of any kind. You will see a big increase even if you just use 10 ah. Figure on getting real world use of 80% of the stated size.
Do this for sure, because your batteries will last so much longer if you really only use about 80% of the capacity, so a 10 ah pack will get you 8 ah of usable battery and still be treating it very nice.
Read up about the RC lico chemisty batteries if you are planning on using them. As I always say, don't store them or charge them in a place you wouldn't build a fire. You might look at lithium manganese, there are several types which are less likely to cause a fire now. They are just as small and light as lico, or even smaller.