Ok, after I read this topic I try that on my Bro's i4 and when I dont know if his suffers from the reception issues very bad like a lot of users (he havent had a dropped call since the day he got it) , yesterday i did a few speed test touching that part and the tests failed or were very slow while if i stop holding that part it completed the speed test normally. Today I cant replicate this, no matter how i hold it does the speed test at the same speed.
I turned off the wifi,gps and BT and the speed test were faster ( like 2300kbps vs 1900kbps ) but it could be just coincidence.
The Phone lose signal when I hold it the wrong way for like 30 seconds, but it still manage to do a few (5 o 6) 1800-1900kbps speed tests with just 1 Bar of signal...
So due to my mixed results I cant say this doesnt work but I dont think it will make a difference...