It showed after some testing, that the maximum Interface speed, by not using the builtin Bridge Chip - is around 10.4Mbps (Mega Bits per second), This makes roughly 1.3MiBytes/s. The limitation is due to the CPU Speed which is running at 175MHz. At that speed though CPU utilization hits 100% - while the CPU is at this level - not many manipulations are possible fluently.
The Graphs on the right show this very nicely. On top - WLan, after that the Ether3 interface - server the 4GB Download was done from, and on the bottom the CPU Load in % during the file transfer. You can see very nicely that the CPU barely leaves the 100% mark..

I have noticed that through PPPoE (Bridge to the ether1 Device) 13.4MBits/s have been transferred. So - it seems the PCI-Bus is costing also some resources to the RB153 Board.