actually, mounting the longer shock more vertical will kill your compression travel.  All things equal, the shock will only travel as much as needed.  The angle of the shock affects it's damping properties, not it's travel. although when you angle mount shocks it will increase the amount of travel needed per shock to not bottom/top out.

Shock effectiveness is as follows:
take your calculator and input the angle of mounting.(say45*)
then hit the cosine (cos on my calculator) and that will give you your shock effectiveness.
thus, a 45* mounting angle will yield a 70% effective damping rate, IIRC.