Pretty much.
Although I don't think colossus procs as much as it did on retail, other things are broken...
That being said, mortal strike is meant to have a formula which is fixed on weapon damage. I don't think it's meant to be affected by seasoned soldier the same way slam is.
Edit: you can find the formula by opening webarchive mortal strike during5.4.8, paste the link on forums, hover over the link and it should give you the formula. If imagine it's the same with other spells too