Wish i could help you out there buddy... but i've never been one for covering up beauty, and one of the reasons why I got an HTC was because i liked the unibody look... would be a shame to hide all that oomph with a case!!
But these things are insanely popular, and even my wife uses one (she's prone to dropping stuff, especially if it has a battery in it!! ), and it's one of those things which is traslucent-flexible-rubberized-plasticky-something... No major gripes with WiFi with that case on, but i'd have to test it myself to form a more educated opinion...
I recently removed the, "traslucent-flexible-rubberized-plasticky-something" - tpu gel case and used the phone for a couple of days and not putting it back in any case again + it's older now, though scratchless except a hot rock on the back removable cover.
These cases do not affect wifi as much as the otterbox ones or leather cases
Here are the ones I have used till now - no wifi problems -