Home Network :: Getting Very Slow Speed Of Between 10-15Mbps?

I have a homeplug network as follows:

6x 85Mbps homeplugs in different rooms, each connected to a 5 port switch

2x rooms contain a PC

5x rooms contain a Squeezebox of various types

Router connected to homeplug

I have realised that I am only getting very slow network speeds of between 10-15Mbps (LAN SPeed Test)which affects Squeezebox's ability to play several devices synched up at a time playing 320Kbps MP3s

I have done some proprietory testing in my home by switching devices on & off & testing network speed accordingly but no significant gains seem to be made. One PC seems to report far lower speeds though at 5Mbps which is 3 times lower than the other PC. They are set up to read/write files from eachothers shared folders (but not simultaneously).1st question is, am I being realistic about the speed of data throughput I expect on a network of this type? I was expecting a much higher speed like 50Mbps.I wouldn't mind investing in the faster Homeplug system but having to buy 6x adapters isn't worth it if I will only get a marginal improvement.