Not sure how to tell, sorry. Here is the flash bracket I use - the RAM mount comes included. http://www.bhphotovideo.com/c/produc...cro_Combo.html
Yeah the loss of light is greater with 2x, and also from reviews I was reading, people said that the image degradation is much worse with the 2x. Initially I was going to get the 2x but that was a deal breaker. I don't notice any image degradation on the 1.4x. I'm sure it's there somewhere but it really isn't noticeable. I guess it's also important to note that Kenko sell a normal and pro range of TCs, which I anticipate would differ by the quality of the glass. I went with Pro DGX http://www.bhphotovideo.com/c/produc...RO_300_DG.html