You could argue that Soulstorm possibly ended in a stalemate... Cyrus said that 'The campaign was a mistake'. Even though we know that the BR's lost it, you could take that as meaning it was a mistake for anyone to try and take over the entire system and that everyone suffered heavy losses.
Ig's couldn't have done that good though... if you ship off 100 Baneblades, and leave yourself with 0 to defend yourself, so you have to build them all again... that just really kicks of bad leadership to me. Add to the fact the mission where you disrupt their supply lines so easily because the routes practically undefended... You've got Valks, why not air lift supplies, or have the factories for the parts near each other so you don't risk ambush on the supply routes?
Thing is, The IG failing to aid the Marines, and also failing to aid the Sisters... If the IG HAD won, attacking the Sisters is Heresy, so the entire system would be up for investigation, and Slaughtering the emperors marines is tanamount to treason too... even if they did win, they'd have lost too as the =I= would be on their asses asap to see why the comander didn't aid forces he should have done.