In terms of detection, sounds travel as well vertically as horizontally underwater. A 1500 feet/500 yds separation in depth is really pointblank range.
that said, no one knows what a MK48 can really do. The data is classified and they were never used in a war. They may be the uberweapons the USN claims or may turn out to be another MK14.
I would think the easiest way to tweak torp behaviour would be to play around with the sensitivity and range of the sensors.