I'm not sure if having a replacement casing will fix that problem but for mine, if I cover the top part of the phone. The earphone area, the wifi signal does drop significantly, and even disconnect at times.

May be a casing flaw, as some people got their casing change and no longer face the network down issue. (I don't have time to claim warranty till now)

gt my sensation SP from HTC@GARDENS yesterday n there's no matte SP yet .  and..now both d bottom edge gt bubbles...=[

Complain about the drop in wifi signal and say got network drop as well. Complain as many things as you can.
Mine is even worse. The casing got physical defects.
Forummer here claimed warranty and they said the network death grip is gone.
HTC changed the casing. It's by HTC.