Just compare OS to Death from Above....
Death from Above deals about 10-15% less damage than OS.
Death from Above is about 3sec channel,
Orbital Strike is about 3 sec cast,
DfA deal about 8.5k dmg and channel ends. Damage is done in 3~ secs. (if nothing disrupted channel, otherwise DfA deal just a part of dmg.
OS deal about 10k dmg after 3 secs of cast and then in time of 9 secs. (if nothing disrupted cast, otherwise OS deals no dmg)
8,5k in 3 sec is about 2833 dps
10k dmg in 3 sec is about 3333 dps
with this change OS is not worth effort anymore
and now its 3,6k dps in 2,5 sec of cast thats = 1440
You forgot that DFA is kinetic damage, so that 2833dps is actually 1982 dps AFTER the armor debuff, or when used by an Arsenal Merc its about a 2323dps
So its 2323/1982 dps (1982 is the number by the higher parsing spec) vs 1440/1656/2400 dps (lethality/engineering/marksman) - and after crits its about 2658/2424 vs 1699/2070/2832 dps
So a Marksman Sniper's Orbital damage is about 6.5% stronger than an Arsenal Merc's Death from Above.