Shall We Consider Reversers in performance Calculations?
For airplanes certified under Part 23, both takeoff (AMC 23.55b) and landing (AMC 23.75f) performance can take credit for thrust reverser (T/R) if it's shown to be reliable and safe. As detailed in AC 23-8C, much extra work would be required to take credit for T/R for multi-engine airplanes:


For accelerate-stop distance: demonstration of full T/R with one-engine-inoperative and in a 10kt crosswind and on wet runway for adequate lateral controllability is required.
For landing distance: the landing distance used must be that with one-engine-inoperative; differential braking needed to maintain lateral control for this situation may negate most, if not all, of the advantage of a single T/R.


For airplanes certified under Part 25, only takeoff on wet runway may take credit for thrust reverser (25.109f). Similar situation for landing distance as with Part 23, 25.125c)3) allows credit for T/R, but only for that with one-engine-inoperative; therefore, the same problem as discussed before applies.