You can marry it to the car, and still have the stock factory map on the car yes. The 1/4 is not that accurate in my personal experience either. But it starts timing as soon as the car starts to move, and stops after going a 1/4 mile yes. And as far as running 93 on a 91 map you are fine. But you CAN NOT run 91 with a 93 map. Unless of course you are going to baby and granny that car to the extreme. The fueling, timing, and all that is different between the two different octane maps. And as far as differences go between the 91 and 93 maps, I could imagine that there is. I am protuned for 91 and 92 octane. And I can feel the difference between the octane ratings on the same boost level.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX