> I am currently downloading several items, and my highest transfer speed is about 11K, which seems very low given that I have a cable modem

How many items at which rate do you download and what's your connection speed?

> and so do the sources from which I am downloading.

Where do you know whether they have cable or not? It's a customizable setting and a second thing to keep in mind is that Cable (and DSL) connections are often asynchronous i.e. fast downloads but slow uploads and the latter is important from your perspective. Well, Cable or DSL is no indication for speed at all. You can get Cable and DSL lines with ISDN speed.

Oh and not to mention that you're maybe not downloading alone from the source!

> Pro or not to go Pro

LimeWire Pro connects to more sources (6 vs. 4) and in theory you'll get therefore more search results with more sources per file in a shorter time. But that's theory! The more important question is: do you like LimeWire? If so then you should consider buying LimeWire Pro to support the continuous development.