Certificated landing performance (AFM) is based on Vref; - CS/FAR 25.125
Operational landing performance (QRH) may be based on a higher speed than Vref; e.g. many manufacturers use Vref + 5 as a baseline, and for autoland. Distance corrections should be applied to the AFM performance for any difference from Vref: – AC 91-79.
In normal operation, residual thrust should be removed during the flare. It will only be a small component of any increased distance (float) in comparison with high airspeed or a tailwind. Lift per se, does not create the ‘float’; the pilot can still land the aircraft at a higher airspeed at the required position, but the higher energy has to be dissipated during the landing rollout.
Thrust reverse is not normally included in AFM performance due to reliability, consistency, and usability issues; - CS 25.125 (c) and (g).
Most manufactures recommend the use of reverse on the ground; however the extent of this is often left to the judgement of the crew based on the conditions. Thus this decision is subject to the variability in human performance.
The problems above could be considerably exacerbated if airborne use of thrust reverse was considered; - CS 25.933 (a) (2), in flight use.
Propellers slipstream may increase lift, but as above, the aircraft can still be controlled to reduce float and land at the required position, albeit at a higher speed. Speed is the dominant issue.
The regulations for propeller driven aircraft differ from jets, but use of ‘reverse’ (discing) in the air involves other risks, and depending on aircraft type, may affect pitch control.
The better defence against landing overruns is to fly speed accurately, avoid tailwinds (higher ground speed), land at the planned position, and select retardation devices promptly. Distance corrections should be made for planned higher speeds, touchdown position, height over threshold, tailwind, and runway condition – wet … contaminated.
CS 25
http://www.easa.eu.int/agency-measur...dment%2011.pdf
AC 91-79 Runway Overrun Prevention