First, any plastisol can be used in salt or fresh. Salt formulas are usually harder.
Usually the softer the bait the better the action .......... With the exception that a harder formula is better for a quivering action ..........
If you make them harder you change the action .......... The whole point is to get the bite .......... You will always spend more on gas getting there than any bait you would use ........... It would be a shame to sacrifice a few bite to durability ..........
Many years ago I found that the sassy shads that worked the best were the 1's that had tails that layer all the way over ......they were also the 1's that didn't last long ...... I would pick over every bin or package to find and fish just those and give the 1's that didn't pass muster to people that wanted what I was catching on ........ Needless to say I still caught more using the same bait.
Question is .......... Do you want to build for durability or to catch fish? Which will create more demand for your lures?