Speakers for gen III JBL (again)

I know this topic has been beaten to death but I would like to clarify a couple of questions. I would like to upgrade the sound without digging into the electronics, i.e. summing device, additional amps, re-wiring. I am well aware that staying with the JBL head unit and amp puts significant restrictions on ultimate sound quality. First, it seems like changing the dash speaker to a coaxial 3 1/2" speaker will work wonders for imaging. It is no clear to me what signal is reaching the stock speaker. I'm sure it is crossed over, but can someone tell me roughly what frequency it is crossed over? If it is too high, putting a 3 1/2" coax is obviously a waste; better to just swap a better tweeter. Second, what signal reaches the door speaker? Is it full range or does it go through a low-pass filter? If it's full range, I imagine replacing with a quality (Infinity ref or Focal coax) may improve sound considerably. Finally, any experience with the 6x9 Focal Toyo speakers? Thank you.