Some things to consider:
If anyone ever makes a 40k fps game, it's going to be up close and personal. Dawn of War was a good strategy game. It portrayed the havoc that could be wreaked from that perspective, but now we are delving into the standpoint of one person, creature, whatever. It might be a good idea to beef it up a little. I have seen mods like Battlefield 40k and although they are done very impressivly and I think they are great, but they don't guite make it feel "crazy enough".
What do I mean when I say "crazy enough"? We'll for instance, 40k is a mean universe, and if Im popping of bolt rounds into the face of some nids I want to hear the loud smacking sound like and screeches like the bugs are freaking from
Starship Troopers 
. When you pull out your chainsword to charge into melee would you like it to appear on your screen with a complimentary sound effect? Or would you like your arms to reach back and look like their grabbing it, then pull the bulky thing out and give it a "REEEEER" or two? Think of quality animation. It's done by exagerating the movements and keeping you interested in every aspect of what the characters doing. I want to be interested in what my characters doing and that's what I think many FPS lack. A boltgun shouldn't go offscreen to reload. You should see your big canned ham space marine arms eject the magazine and slap an big fat new sickle mag in! KER *****!
. Oh yeah...thats the good stuff.
I'm just saying in all that most games that go are fps don't show alot of "finesse" or "pizazz"...and if I play a 40k game I want to play it and think HOLY CRAP that EXPLOSION just threw my character back 10 feet, now I have to go back and pick up my bolter out of that rippers MOUTH. Well maybe thats extreme...but anythings possible my friends
GET INTO THE GAME!!!
As for if the game can "handle" such a thing...we'll I don't care if it has graphics like The Elder Scrolls Oblivion. Sure, if you want to make it able to pump itself up to play with 900000000000000 polygons per squig then by God its your right to. But honestly I'd rather play with a billion low polygon enemies all running smoothly. If all my enemies look as good as World of Warcraft (whose textures make up for everything polygon) I would be just fine. It just means that the designers of the game can focus more on the massive battles in Warhammer 40k.
And honestly...Would you rather fight a 2vs3 battle high polycount? Or a 200vs200 or something battle low polycount. Starsiege Tribes supported multiplayer battles of over 200 people years ago, so I'm sure something could be accomodated for today. There are always possibilities. Concider it at least
That's my 2 Imperial Credits. Cheers~!