If your going from a station to a RES site a lot. Or you have powerplay reasons to stay in one system for a while. Or you just like to patrol your home system. Then sell your regular FSD and buy a special system-drive FSD. This would allow faster RES/BACON turnarounds for re-arm and re-fuel. Your ship wuld be faster and more agile in supercruise. This would make interdictions easier to initaite or avoid. It would allow for a record breaking atempt at the Hutton run. The downside - no hyperdrive, if more downsides required it makes more heat than a regular FSD.
The presence of a player buy-abe module explains how system security seem to an improved FSD already fitted! In fact it justifies this, the reason system security are more agile in supercruise, is because they have better FSDs.