BOAC,
Are you testing on a single PC or with 2 PCs? It is possible that different PCs will provide different results depending on their configuration, particularly with Java (which a lot of the speed tests use).
Could you specify exactly your network setup?
I conducted some tests with my home network, using most of the UK sites listed
here.
Over 6 comparisons, 2/3 were virtually identical between PC 1 (copper cable to Netgear router / WAP) and PC2 (11 Mbps wifi, 128-bit WEP enabled).
The testing wasn't absolutely rigorous, insofaras I tested sequentially and there could be some upstream caching at the ISP that was affecting the speed results, but as I tested wireless first, the effect (if present) seems marginal.
Try running several tests from different providers and see how the results go.
If testing with a single PC, ENSURE that you clear the browser cache before running the same tests again.
Dan Elwell's test might be helpful in locating any choke point (if one exists).
Finally, yes a MAC address can be detected by the radiated wifi signal - provided that one has a wifi packet analyser.
SD