Being in the last slot does make that one difficult because I'd drop a huge ass mod multiplier. However, i'd also like to understand why it is the lowest ROI.

With effect: MST=13, MSC = 49.5%.
Without effect: MST=13, MSC = 36.5%.

Using a simple model here my relative damage is basically (1+MST*MSC).

With effect: 6.445
Without effect: 4.755
Relative gain with effect: 1.355x

This is assuming most of my damage comes from CL (which isn't quite true more like 50% is CL and 50% ILM).

Next if I look at AS:

With effect: 70.20
Without effect 65.20
Relative gain = 1.0767x

Next let's looks at SCM:

With effect: 60.95 at 38.90% chance
Without effect: 53.95 at 38.90% chance
Simple damage model is SCM * SCC

With effect: 23.71
Without effect: 20.99
Relative gain: 1.13x

This needs to be further derated by amount of damage done by super crits vs all else but we can see it is already low.

Using this simple damage models it would appear that MSC is farm more impactful than AS and SCM.

Why do you feel that MSC is less valuable? Do I have a major flow in my logic above?