Thanks Billy, some good advice/insight there for me.
I've actually come to the conclusion that I have more slow-twitch muscle fibres in my chest than the typical male. Just a theory, but I've never really done a high high-rep, high-set, short-rest, heavy-weight workout for my chest. It's always been traditional 6-8 rep/3-4 sets/long rest/heavy weights. So it might well be worth testing that theory for a few weeks, or mix it up as part of my routine to keep my chest guessing. This theory would certainly explain a few things though. In fact, your 1/2 hour cable exhaustion routine at the end of your workout seems to suggest exactly that for your own chest, combining traditional with high-rep.
I have been incorporating some wide grip lifts into my workout since that was suggested here early on, and seem to be seeing some results. So I will focus a little more on that now for a bit and see if I see even more dramatic results.