One of the key features offered by WTFast/LagKiller is the ability to change the ports you use to play your game. ISPs often shape ports differently, so while they might be throttling 3724, 80 or 443 might be better for example. WoW does still use port 3724, but that is only between the WOW game server and the WTFast server; once the traffic goes from the WTFast server to the game client/computer, the port changes - thus impacting any traffic shaping policies that your ISP might be running on ports.
In addition to helping to deal with any traffic shaping policies being run by ISPs, WTFast can also help by improving the route that your game traffic takes to get to the game server. Instead of taking the default data route, you can pick a different one. It is just like taking a different road when you know there is a traffic jam on the road you would normally take.
Together, both of these things can have a big impact on your game connection. If WTFast is unable to solve your game lag problems, it could be because your ISP is having issues on their own local network.
If you provide your ISP with more information, with good detail showing testing you have done with WTFast, it can be helpful. i.e. if you can tell them that you tried 5 different WTFast servers and the connection is crap with all of them (and provide tracerts to each of the WTFast servers you tested); you would have a good argument to present to them, indicating that the problem is with their local network, not something outside of it. Wut? We can't blame Blizz?!??!?
ISPs are not likely to admit they are the problem, so you need to give good evidence to show you are not imagining those 1k lag spikes.
P.S. To do a tracert, Start > Accessories > Command Prompt, then in the new black/white window, just type tracert [insert name of WTFast server here], i.e. tracertca1.wtfast.com. To copy the data, right click on the window, select "mark", then click+drag a window around the data you want to copy, after it is select, press "enter". This will copy the data to your clip board. Next, just type CTRL+V to paste it where you want it, an e-mail, document, etc.