Say total piping is about 4 feet.
2" diameter pipe roughly has
3.14x1x1x48 = 150ci
3.14x1.25*1.25*48 = 235ci
Difference in volume 85ci
a 1.6 at 7k rpm flows about 170cfm
Add 6psi of boost to that so you'll increase it to 200cfm
85ci to cubic feet is 0.0491898cf
200cfm=3 1/3 cfs
3 1/3= .0491
1 = x
So theres ur answer, the difference in spool at 7k rpm is .0147 of a second.
Since you wont be spooling up at redline lets add something more valid such as half of that at 3.5k rpm. So say 100cfm
1 2/3 = .0491
1 = x
At 3.5k rpm you'll spool with .0294s slower with the bigger piping.
Looking at those numbers the only benefit i see with going with smaller piping is the ease of making it fit.