Thanks!

Whatever it is, it seems to be case of poor quality management! It´s not like that this is very hard to notice.

I´ve tried the following:

Reset the device to factory setting; in the setup manager I refused the option to install other apps.

This seems to have had a positive effect. 

It is still far from perfect or even acceptable, but next room now has good bandwith and the drop to the other rooms is significant, but mostly at least seems to be a little stronger than before (mostly above 1 MBit)

Here´s the interesting part: The upload bandwidth is good! Let´s say I get 1 MBit/s downstream downstairs, but I still get at least 2 - 3 MBit upload stream at the same time. Very curious result I´d say, a higher UL than DL ratio. 

Also the Wifi Analyzer app shows me the same WiFi strength (dbm) in all rooms. So the signal is there?

Well, a user from a German board contacted Archos support. They claimed that only a few devices had that problem. 

So I sent back the first one and ordered a second 97 HD from a different vendor.

What can I say?

Same old problem!

Meanwhile German amazon customers have also started writing reviews complaining about the WiFi performance. 

the signal does not vary much on the ARCHOS. I suspect the returned values for the sensibility are erroneous, maybe to hide the low performance of the WiFI on the device.

That´s true. The meter might show 4 of 5 rings, but that has got nothing to do with the data rate. You may have both 12 MBit or 0,9 MBit while the same signal strength is indicated....

As to the metal backside: I previoulsy had a (cheaper) I.onik tablet with a metal backside here - which had a really nice WiFi performance...