As of references, since most fluff is from the marines codex, where else would you expect me to find references?
The Black Library, for one. Old White Dwarfs, the Main Rulebook, Imperial Armor are other examples.
However if you like, go read Brothers of the Snake (novel) or the Soul Drinker series (especially the third volume) to see how pathetic IG is compared to SM.
Go read Gaunt's Ghosts and watch I.G. shred Chaos Marines. It's even more canon than either of your novels because GW has produced figures for the Ghosts and has background to the Ghosts in the I.G. Dex; they used to have rules. Still, I think the Ghost stories are exaggerated. A grain of salt is needed when reading black library novels. Each author wants to make his or her protagonists 'awesome' and so we get them doing things way out of proportion to where their abilities should be. This somewhat applies to Codices, too. The best and most balanced view is where the army in question is secondary to the main characters. I rather like the presentation of the Guard and Marines in Esienhorn, for instance.
It's not that fact that I.G. are inferior to Marines I am arguing, but the fact that your statement implies they spend their time whimpering for Marines to save them, which is very false. Read any story with the I.G. as protagonists or even secondary or tertiary characters and you won't ever see them doing that. Guard are professional and competent and the fact that the Imperium's greatest conquest, the Macharian Crusade was done by the I.G. completely is proof of that. Further proof comes from the fact that the I.G. fight most of the Imperium's battles and are the backbone of the Imperial military. If the Imperium's core army was sucky and pathetic, well, it stands to reason that the Imperium would have crumbled before 10,000 years.
"our prayers have been answered! Here come the SM!"
How do you know that they were praying for the Marines specifically? They certainly were praying for salvation from the Emperor. Most likely, they prayed that the Emperor would send someone, anyone to save them. It was the fact that help had arrived, not that the help was specifically Marines is what caused them to rejoice. They probably would have reacted with similar jubilation if a massive Imperial Navy flotilla appeared with transports loaded with Cadians led by Creed.