Yes,thanks to Bloody Angels Federation learning would be rather bloody!
But - they would learn and survive.
I always laughed and crossover when IoM stomp ST easily.It would be crippling conflict for them,and Federation could still survive.
P.S Merry Christmas !
They would have no choice after facing Marines.
Still should have made better infrantry.Or,to be precise - dudes which would coordinate teleporting shit into their enemies.
If Federation let them.They had plenty of weapons to kill in brutal way,only decided not to use it.Imagine teleports used not to move dudes,but teleport bricks into Astartes heads,for example.
You knew,that without Sanguinius and his Chapter,Horus would win? you just gave WH30 Galaxy to Chaos.
Unless...send one chapter who would fall to Chaos ,too.
Preferably - near Cardassians,so they could not profit from entire mess.