My take on it would be that when you consciously flex, say, your arm, you are "artificially" creating more weight by making your triceps contract a bit, which adds to the pulling force on your biceps. If this is what you are doing, then more weight or flexing throughout each rep may have a very similar effect.
However, it could be more difficult to keep the "flexing" force consistent throughout the exercise which would make direct comparison difficult from one workout to the next.
I do some flexing at the start of the 15s (and keep going to 20 reps if possible), but as the load increases I forget about flexing and just ensure I can get the reps out. Also, as I have seen mentioned in other threads, the 15s aren't going to make much difference to muscular size. As long as you get a good burn going, the exercise will be doing its job.
Just my 2¢
Lol
PS. Just remembered that Vicious has some very useful info on techniques to add metabolic stress in his Pimping My HST article. You've probably read it but, if not, it's definitely worth a read.
