Posted this in Eliroos thread on reddit

Also went ahead and got some numbers for you,
Fuma: http://puu.sh/dxBfd/54aad59b4d.png Raiton: http://puu.sh/dxBiX/7cfc1c9756.png
In the parses you can see the damage variance on fuma is 626~673 non crit (not counting crits because I do not have enough), while Raiton is 702~746
So for the high end I lose 120 dmg per mudra (746 - 626), while on the low end you lose 29 damage (702 - 673), so the damage variance (for the small sample size I did anyways) is 29~120 dmg lost per mudra.
If you average the numbers in the parse out you get 718.4 for raiton, we shall round up for worst case scenario, 719, and 651.8 for fuma, we shall round down for again for worst case scenario.
719 - 651 = 68.
On average you can expect to lose 68 damage per mudra based on the numbers I presented.
Using the numbers you showed in your original post 10 fuma (3160) vs 10 raiton (3600), you would lose a total of 440 potency over that course of time. If we use the average we are losing a total of 680 damage (1200 if we use the highend number). Using spinning egde (again assuming the worse case scenario, the 2 gcds that you gain from using fuma will be spinning edge), which averages 390 for me, the 2 GCDs you would gain (4.6 seconds) will be made up for damage wise, but not potency wise. 390 (2 spinning egdes) x 2 (2 gcds gained from using fuma) = 780 but, 440 - 300 = 140, so while you may lose 140 potency, the damage can either be made up for a gain of 100 damage or a loss of 420 damage.

This is all assuming the worst case scenario, if you end up on anything other than spinning edge you are most likely looking at a DPS gain over raiton.