
my N times for TMX and D76 1:1 are:
N = 8 min
+1 = 10 min or 25%
+2 = 11:30 or 43.7 %
1 = 7 min or 12.5%
2 = 6:30 or 18.75%
Your Prof is mistaken, IMO, as far as determining basic "N" developing times. But some situations can involve extreme measures that may involve changing the agitation schemes, developer dilutions, etc...but for basic "N" compensation, no, as far as my own work goes anyway.
I have developed negatives exposed to very low SBR (1:4) for 3x my "normal" of 8 min for a 24 min developing time, that was a 300% increase in development even using standard inversion agitation at 1 min intervals. You can see the result of that negative on my Flickrsee "YellowPoplar Leaves".