finding any information on that topic is a crapshoot, everyone seems to have their own opinion. its highly circumstantial (equipment used for source, cable brand/quality, interferences, television used, total length of cable, etc) but some people report no visible loss at all (especially in short runs like joining two six foot cables ) and some report that they see "sparklies".
i've never heard of there being an official test to get any numbers or comparison images but any extra connection or distance over 50ft (if i remember the max distance right) will introduce some sort of degradation. having a very long run and a splitter could possibly cause more interference than a coupler on two short cables in a short run however in your...