My house is fully wired with Ethernet, and it's all cat 5, which still works fine at speeds of 1GB, and will almost certainly work fine with 2.5GB when those switches come down in price.
In addition virtually every wall outlet plugs into a relatively inexpensive hub which feeds the receiver, the apple tv and one or two other things. Everything works fine, there is no packet loss and changing cables around, particularly with the short cable run in most houses (300 feet is a short run) won't make a difference unless one of the cables is broken internally.
Remember, your DAC does not directly consume the output of Ethernet (which is an analog signal). Inside each piece of equipment with an Ethernet jack is an Ethernet chip that decodes the signal and converts it to 1's and 0's. So changing wires won't make a lick of difference.
If the sound isn't quite where you expect it in your equipment, it probably has a lot more to do with the acoustics of your room or the recording than anything.