As HappyDaze said, this is probably one of the few systems alot of people isn't too terrible in.
Of course, there are still going to be some issues.
1. How would you rate the groups knowledge of 40k? I would assume the large interest means you got quite a few people who always wanted to play 40k rpgs, just didn't like the acolyte/rogue trader setting. Are you considered the most "knowledgeable" in the setting? My point is, it really undermines the GM when players start correcting them on minor details. So be prepared to just tell them whats what, or make use of your knowledge of the setting.
2. Combat speed. 8 players (and 1 GM) means that every time one person gets their turn, they have to wait for 8 more people to act (remember the GM does something as well), before it is their turn again. That can be a long time, especially if people are unfamiliar with the rules, or spend time trying to figure out the best course of action. I wouldn't go so far as to implement a time limit, but I would ostracize those who take too long. In combat conditions, even the best warriors in the Imperium don't have 30 minutes to figure out the optimal course of action. Encourage players to know what they are doing by the time their turn comes around. You yourself should also be prepared to describe what your hordes and NPCs are up to. If there is an argument over the rules, just roll off on it, or find some middle ground and apply that. The key is speed. You don't want players pulling out laptops and starting up <internet addiction substance here>.
3. Spotlight: this is obvious, just try to make sure everyone has something special about them, or at least are doing something. If someones just sitting there, ask them what their character is doing. part of this, is you don't want to ignore someone for 30 minutes, and then get to a point and ask "Hey, wheres Brother <generic name>us?"
4. Difficulty - 8 space marines eat danger and crap victory. If things seem easy, ramp it up. For any pre gen scenario from FFG, I would at least up the horde mag by 25%, and add more NPCs to smaller encounters. Don't be afraid to kill a character, there are 7 more. And if they recover his GS, he gets a bonus (2 if he pulls off that heroic sacrifice deal)
Other tweaks: I might consider upping their base cohesion, just to allow more players to actually activate squad mode abilities, maybe +2 or 3. Also, I would probably try to make it so that each character has a diff armour history (just for the sake of variation).