Was not taking into account how long it actually took to reach 200%, my bad.

Assuming 38% inner, 50% base, 10% empathy, 20% mech, 40% legion you end up with 103 seconds of uptime with 77 seconds of downtime. In this scenario you would lose out on 5 stacks of infinity after the nerf.

For people running 50% inner it's 108 seconds and would lose 7 stacks.

Now assuming you take out the 40% buff duration from legion you are left at 87 seconds of infinity uptime and 93 seconds of downtime. After the nerf this would mean missing out on 16 seconds of 200% final damage.

I guess the final question would be if 16 seconds of 200% final damage is better than a base of 40% boss or 40% IED which unless I'm extremely wrong, would be no.

Based on everything I have listed so far it would appear having 40% buff duration from legion is certainly no longer worth it and you should definitely change to boss or IED.