My N800 has great wi-fi sensitivity.
As for speed, it's true that it depends on signal strength. The wi-fi driver negotiates a transfer rate which mostly depends on the signal quality (which is again related to signal strength). If you have 'Wifiinfo' installed you can monitor the current negotiated rate (from 54Mb/s and down all the way to e.g. 2Mb/s). Or use iwconfig from the command line.