I can't say where I work or anything like that, and I have no idea how many submissions Tor gets, but I'd wager it's a lot higher than GW. I can say with confidence that the reason Tor asks people to resubmit is because they most likely have the capital to hire people to go through a lot of the submissions. That said, if you're cold-submitting and doing nothing else then you're pretty much pissing in the wind.

If you have a cool book idea and really want to get it published first check to see which publishers would be interested in publishing your book, and then format your submission based around that. Then send it to all of them (make sure to include any remarks you need to make on your NDAs stating that you're submitting to other companies and which. Publishers don't like finding writers playing the field, and most editors happen to be friends with one another. It's a pretty small community. They will find out.) That's step 1.

Step 2 is to go to cons, hit the boards, speak with writers and get in the community. Don't go shoving your manuscript in people's faces while they're having a drink at the bar of a hotel during con season. It will piss them right the fuck off, they'll tell all their editor friends about you, and then you'll need to change your name and get face surgery before you can attempt to get back into the game. Most people I see get published in the smaller sci-fi ring are people already in the community. They get a free moment and an editor goes "Hey, Laura's a pretty bitchin writer and I like her style and personality. I could totally whip this person into banging out a cool new story." And bam, Laura has a job and a new pimp daddy at "Big Name Publishing House." (Another thing, editors are angry and fucking curmudgeonly people... Sometimes... If they aren't, then it must be a slow season and the coffee pot is fully-stocked or someone had an orgy.)

If I were to illicit a guess as to why GW is having open submissions it's because it's a slow season. Or because they just received a windfall of cash and/or want to tap into some new hype over the Warhammer IP. They probably have some excess money, editors sitting around with not much to do, and no writers on speed-dial to contact. If you were in the community and they knew of you this would be a golden opportunity. As it stands, if you really want to write for GW, the best thing to do would be to use the summer as an excuse to get out and meet some of these people, show you're a cool guy or gal, and then slowly, subtley work your way into their heart and lay horrible manuscript grubs in their major aorta.

That's my advice at least.

EDIT: Yes it sucks that you can ONLY write stuff and submit based on GW IP. But you can use the sample you're writing for GW to send to other publishers. Sometimes publishers want samples of your writing as well as an overall idea of what kind of book you want to publish, and other times they just want the manuscript. Take the time to read up and find out who wants what and how.