It appears to me that you are equating N minus and N plus with film speed adjustment. N minus and N plus are indicators of development adjustment rather then exposure adjustments. Contrast is determined by development.
Originally Posted by kjsphoto
Having said that one would normally make adjustments to film speed when development times depart from "normal". For instance in a N minus one (appr. SBR 8) I would decrease film speed from normal by 1/3 to 1/2 stop to support shadow detail (give more exposure). In a N plus one (appr. SBR 6) I would increase film speed by 1/3 to 1/2 stop to obtain greater effect on the negative density range (give less exposure).
As Gary stated split bath development is a compensating development and would normally be used in situations of excessive contrast. I personally would not use it unless and until I exceeded SBR 9 (appr. N minus two). There are many "tools" in photography. The matter is using the proper tool at the proper time. One would not normally attempt to fry eggs with a ball pein hammer.