Not much difference other than the XFI would be referencing the MAP sensor to turn it on. If the MAP sensor went bad you would know it because the car wouldn't run very well.
Yes, you could do this if the NOS controls are enabled in the Classic FAST when they built it. I have seen a lot of Classic Buick boxes that don't have NOS control.
IMHO....if it was made by human hands it can fail........but IMHO.....it is probably more efficient....and visually appealing to let the FAST do it....