a tilted scope is pointless as your hit zone would only be at a VERY specific distance. you would need a range finder to use it properly, and you would have to use extreme compensation for anything not at that perfect distance.
All our guns have elevated optics, its not a problem.
FYI, airsoft guns are nowhere near accurate enough to worry about the sight offset.
http://www.theboxotruth.com/docs/bot18.htm for some fun info