Hi succi,
Normally if the magnetmix files download at snail speed, it's pointing to an ISP shaping p2p bandwidth. I was expecting them to download slowly for you:o
But they don't so...try changing ports to one in the range I posted before. Since your connection's good & you're not firewalled, your setup can't be too bad & since you got good speeds from magnetmix, it's probably worth trying a different port before giving up. Have you tried downloading at different times of day (& night) to see if speeds pick up/drop off at certain times?
How do you have your router set up (UPnP enabled or manually forwarding a port) & what brand & model is it?

Don't know if you've seen this sticky, but it's got some good tips for optimising LW's performance...might also help you to improve your speeds a bit.
Configuring LW for best performance