No,that didnt explain it - but it turns out to be pretty easy to flash to openwrt.
if anyones interested, I'll point you towards to instructions.
Openwrt is a fair bit sparser than the glorious interface of dd-wrt - but the power meter does seem to work and I can adjust up and down.
using wifi inspector, it doesn't look as though received power has increased much both at my pc next to the router or the HTPC downstairs, but, and here's the good bit... sending a large video file wirelessly from the PC (next to the router, running a 802.11G network ) to the HTPC (downstairs - running on 802.11n) took around 80 minutes or so under dd-wrt, but using openwrt its
> around 20 minutes if I wirelessly connected to the router from the PC
> under 13 minutes if I connect via cat5 from the PC
This is a good result, since now for the first time I can stream from upstairs to down and vice versa, and might even get the xbox360 extender working at a decent pace.
No idea why its so much faster under openwrt when the signal seems to be no stronger.
Transfer speed was around 10 Mbps, but given I can get 20 to the internet wirelessly through the PC's g network, something is still not quite right...
but its a good step forward. Maybe I'm pushing the boundaries of the little Cisco usb dongle which has no external antenna thats in my HTPC downstairs until the better dongle and antenna arrives.