Interessting.
I also own a Mini with silent-upgrade, which also does'nt work properly. In G5 it is a normal A1027.

WLAN is WEP-104 (also tested with WPA/WPA2), Band 2,4 GHz, Bandwith: 20MHz, Wireless 802.11 Modi: b,g,n, Channel 1. No other foreign WLANs here.

If Mac is very near to Router (1-2m) then a partly stable connection sometimes is possible. But it is impossible to use Envoy - other computers can be seen and connected, but data-transfer is impossible or terrible slow or has a lot of interrupts.
Needless to say, that LAN and Atheros-WLAN works flawless with same settings.
This behaviour occoured from beginning (since first Mini was supported by MOS) and nearly all of my friends have same problems.