I've just replaced the 4A G5 FSD in my Cobra 3 with the SCO version, and I get a 3.5Ly increase in jump values. With the speeding up of in-system travel, in this case it is worth it.
If I do the same with the 5A pre-engineered FSD in my Python Mk1, the difference is only 0.1Ly, so I have put the original FSD back in.
If I fit the 5A G5 SCO FSD to my ASPX, which also has a 5A pre-engineered FSD, I get a 3Ly improvement in jump values. With the added advantage of reducing travel time to more distant planets whilst exploring, I am going to use the SCO FSD from now on.
Of course, if I get the Python 2 (with my in game only ARX), I will have to buy and engineer another FSD.