http://www.trustedreviews.com/Sony-Cyber-shot-WX5_Digital-Camera_review_test-shots-iso-performance_Page-5
I think the results are something that we should trust. Not because the website is called Trusted Reviews, but only that the pictures speak for themselves! ;-)
So, as the reviewer has commented, at ISO 400, noise starts to jump in, at 800, things go downhill and at 1600, the picture is just "unusable", IMHO:-
(ISO 1600, Sony WX5 w/ 12MP Exmor R BSI 1/2.3" CMOS Sensor)
Read on the Last Part.