$425?
I have the Cardas and Blue Dragon cables on my Senns. Haven't really noticed a difference, though the cables are nicely made. Think of it more like putting chrome trim on your car's engine. Maybe the chrome will help with heat dissipation (or whatever) but it'll still be the same engine as it was before.
If you have a spare $400, put that into a desktop amp, turntable or another pair of headphones. Those will give you real, tangible benefits. They're not just dress up.
If you're curious if cable tweaks make a difference, experiment by yourself. Here's one you can try: buy a second HD-650 cable from Sennheiser for $12. Take it to the cryo facility in your city and have them cryo it for $2-$5. They might do it for free if you ask nice. Cryo doesn't cost hundreds, it's actually pocket change. After you cryo the $12 cable, switch back and forth between it and the stock one. If you hear a difference, you saved over $400. If not, you'll have a spare cable. You can also use the connectors on the spare cable to build your own cable of your choice. Good quality wire runs a few dollars a foot and, again, you can have it cryoed for the same cost as a Big Mac. There are plenty of build guides and information on making cables in the DIY Forum. You don't really need any special tools or education to build cables, either. Then, for 10% (or less) of the cost, you can hear for yourself whether cables make a difference.