It makes no change whatsoever. It's silicone people, not metal. If your left hand is covering the GPS/BT/WiFi antenna reception will be affected..
actually you don't know that and i'll give some insight why i suspect it might
carbon is used as the black coloring agent in paints & plastics - i don't know about silicone but there's no reason to suspect it isn't as it's the easiest choice. Carbon will conduct electricity as well as shield radio signals. Carbon fiber is beautiful if you want to shield something from EMF or radio signals.
but black paint will degrade radio signals, and i had a suspicion black plastic, rubber or silicone cases would as well.
If jonathan75 confirms that's what he tested, then maybe even removing the rear plastic case will give us some idea if the problem is antenna related or otherwise
PS - for the hey of it, used the signal strength reading under settings > status, i pulled the black plastic case cover off the back
in about 30 seconds, signal went from -111 dBm to -81 dBm then climbed back to -105 dBm - first time i've had a reading here under -94
put it back on, it jumped to -111 dBm again
waited 5 minutes, did it again, this time it dropped to -105dBm
not a lot wrong with a gain in -6 dBm in signal strength
not conclusive but be interesting if others tried it and reported what they saw -
it takes a bit for the new reading to be reflected - i don't think the phone pings the network constantly