addint a string retainer will do nothing to increase tension ... given string length (bridge saddle - nut) + given string diameter + given string tension = pitch
it does increase the downward string pressure on the nut, which eliminates the potential for the string to pop out of the nut while playing. the sonic properties of an open string change slightly with the added nut pressure, but not significant enough to write a thesis on it
Fender's location for the extra string retainer is a little puzzling - they have put a screw right at the edge of the weakest part of the neck. why?? it makes more sense to locate it a little further back towards the high string tuner, but not so close that changing strings is a total pain.
the only strings in a 4+1 configuration that greatly benefit from a string retainer are the middle three. another reason to love the Hipshot 3-string retainer is that you can bring the string break angle even across the nut for all three strings
all the best,
R