I've just read NASA Technical Memorandum 4541, which is a discussion of the reduction of drag and increases in aircraft performance at "dangerous" speeds, such as landing with high-lift devices deployed, go-around situations, etc..

Referring to twins, with nacelle-mounted engines:

One of the items tested was to increase (downward) the inclination of the nacelle (thrust line) during these times, and it did indeed reduce drag and improve performance, especially at low airspeeds where the wing was pitched up, and the propeller was running at high speed. It was even suggested in the report that a moveable nacelle might be a worthwhile improvement to existing twins.

NASA has the best web site on the planet, for those of you who don't already know. There are TONS of free images, drawings, movies, technical reports, and other info, freely downloadable. One of the best sources is the Langley Technical Reports Server, at http://techreports.larc.nasa.gov/ltrs/