I went to the map of elevations across Hebes Chasma. (

) The elevation difference of 8000m occurs over a horizontal distance of ~ 0.5 degrees latitude. On mars, 1 degree is ~ 60 km, so this is a slope of 8 km in 30 or about 15 degrees.

In the movie and the ESA still pictures, slopes appear to be more like 30 degrees, so I suspect (but can't prove) that a vertical exaggeration ~2x is buried in the displays somewhere, to make them look "more realistic" or "more dramatic" (less real, and less scientific)...

For an image of Olympus Mons http://www.esa.int/esaMI/Mars_Express/SEM9BA1PGQD_0.html , they applied and declared a V.E. of 1.8x

In another study of Melas Chasma, ESA used a V.E. of 4x!
This study noted that slopes over 15 degrees are relatively infrequent (<5% of the area)
Clearly, each image or study needs to declare the V.E. used...

You might find THISstudy of slopes on mars of relevance, though it predates the MEx mission:-

http://astrogeology.usgs.gov/Teams/Geomatics/geodesy/P22D-10.pdf