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 situation short of running an entirely new cable the right length (which is the "optimal solution" this is your only real choice.
if you're using a decent shielded cable at a reasonable length under the maximum and you get a decent coupler i would say you might be fine. keep in mind you can always buy at a local shop like bestbuy, walmart, etc and just return a cable & connector if it doesnt work how you want it to.