Vilas sir, I have no idea. Instead of engine core efficiency my initial thoughts would be towards overall aerodynamic performance, namely:
a) air mass density / compressibility - IAS-TAS-Mach relation at that region (some sort of an optimal point)
b) Rudder effectiveness / spoiler use related to a)
c) speed polar of the wing, as for different weights the OEI speed will be slightly different so perhaps reaching some sort of optimal alpha point with relation to max Cl/Cx or min Cx.