The vertical has had a number of runs since the first post.
I have not actively compared its performance to my end fed antenna. But it seems to be no less efficient as I have worked a few JA station on 18MHz when using it.
The design remains similar with the exception of adding a 1:1 current balun. This was introduced to isolate the coax from the antenna system, as I noted some variation in VSWR as the coax was moved. This was discussed here on the SOTA reflector (look for post #7)
I have remained with the3.7m radials but I have increased the number to 8 (yet to be tested).
After reading An experimental look at ground systems for HF verticals (second link below) I may increase that number to 16.
Naturally the internet is most people go to when it comes to research, to date I have turned up three items of interest
I have also found some interesting reading by Rudy N6LF, Rudy has done a huge amount of work with antennas. two articles caught my with regard to radials for vertical antennas;
First one, on this page look for Series of QEX articles on ground system experiments
The summary of these articles are covered in this document Download QST March 2010 Ground Systems
The second one looks at radial length for shorter verticals, one this page look for Are the lengths of radials related to the height of a vertical? to find the article Vertical height versus radial system