NGP-ET passenger has a halt for every 10km on an average. And given that it's a passenger, it'll be routinely held back for overtakes by other trains. The P-7's superior acceleration will be of good use here.
My point is, the P7 has better acceleration and higher MPS than a P4/WAM4, but the higher MPS is of no use because there are very few stretches in IR with MPS greater than 110-120 kmph. A WAM4, or even a WDP1 can comfortably hit 100KMPH+ (more or less). So the higher speed is of no use. The better acceleration makes sense if the train has frequent stops, not for non-stop trains. So, a P-7 is better than the others...
more... for trains with many stops and relatively tight schedules, but for Super-fast trains, it won't make a difference which loco is used, unless it goes in some high speed section.