I have a 5.8Km link running 2 routerboard’s with atheros minipci cards and 2 x 24Dbi Hyperlink grids, I am getting 24Mbps throughput when testing with the bandwidth-test.
I think that that is pretty much the best throughput that you can expect to achieve with 802.11g, even when I test on a test bed with the same config but a few metre’s apart I achieve the same sort of throughput.
Also turbo mode will only run on very short distances. If you need more throughput perhaps you should test n-streme.