First thing I'd ask, did you set the TcpAckFrequency to 1? You should see a significant improvement in latency in TCP games like WoW if you do - depending on your server. On Draka, my lag went from 150-200ms down to 40-70, as measured in Dalaran at peak times. On another server, my lag went from 40ish to around 10ish (!).
That's the biggest difference the Killer cards will make for you; however, it can be done with a hack, so the card isn't needed for WoW.
It's hard for me to believe that if you use the Killer firewall, voice chat, and optimize your network settings, that it won't make at least a small amount of diffeence at least in a mediocre computer. Indeed, it was my experience in such a computer that I did have a slight benefit in feel of WoW and using the hardware firewall slightly improved it again, even though I did the TcpAckFrequency = 1 hack independent of the card.
Now that I've said that, I really think you did bigfootnetworks a slight disservice by using WoW to measure their card; it is primarily a UDP enhancing card. While in the past they made claims about improvements in WoW, when people started pointing out in their forums (me for one) that close to the same results could be achieved by 'hacking' the registry, they backed off thier claims for WoW. They have said many times the Killer is primarily for UDP protocol games.
Bigfootnetworks realy though in my opinion has done themselves the biggest disservice; they released the Xeno Pro too soon. They built the previous cards (at least the M1) with the idea in mind that users could write apps for it, but when that didn't happen they didn't do much about it.
I'm no network guru - far from it. However, it seems to me that network connections are a tricky business, and it is easy for a driver or hardware to go haywire if things aren't perfect. Bigfootnetworks didn't, from what I've read, do everything they could to make sure that as little trouble would happen as possible. People are having trouble that shouldn't be - as your attempts on multiple suystems showed.
About your comment that 'we are tired of spending needless money' - what does that even mean? 'Needless? lol none of it is 'needed', it's about gaming. however, I love hardware and building systems, so none of it is 'needless' to me; I'm NOT tired of spending money on it, lol.
My recommendations for WoW - hard to justify this Xeno for it; if your guild uses Teamspeak, it might be more attractive. Use ALL it's features for max effect. Instead though, I'd say do the TcpAckFrequency hack (there are multiple guides on Youtube and other places), and if you have 4 cores or more on your processor, optimize WoW to run on all 4 cores - or whatever other config suits it on your machine. There's a guide for that on the WoW forums.
Prioritizing your traffic might help, too, but I don't run much when I game so it's not been much help for me.
If you are running UDP based games, I suggest reading more on the Killer cards before you decide against it. However, I'd say either buy an older card (K1 or M1), which are more expensive but less buggy, or wait a bit for the bugs in the Xeno line-up to be worked out.