you're right, distance over open water is very hard to judge, if there are features, islands etc then the estimation is easier.
Go on a boat out the other side of the Needles and try to judge 5, 10 or 20 miles back to shore, now that is difficult.
Frothey, what shockleader are you using?
Some are quite stretchy and you may be losing some inertia on the stretch, so that may be worth investigating.
You really need to speak to a casting instructor, explain any injuries and limitations on bank space and they should be able to increase your distance.
Failing that, come on a winter trip with me, Stu and Wol, we've managed to increase peoples distance and accuracy immeasurably.
Most rods will do it, most reels will do it, most rigs will do it, it's the technique which just needs fine tuning to gain those few extra yards.
Lead size needs to be adjusted according to rod capability AND arm speed, a good observer can point out any little improvements and then it's just a case of practicing.