thanks for the vote of confidence, bro.
I fish mid-length, mid-line-weight glass rods in the hill country - 6'-8' 5/6-wt.
Can't think of a reason not to fish a 9' rod at the coast, but there's no reason to not fish a medium-length rod, either.
While you can get up better line speed with a longer rod with a stiff (fast) tip, I've noticed specifically with progressive rod tapers (tip a touch on the soft side), such as my Fishers, which are magic wands for accuracy, stiffer medium-length rods will cast every bit as far.
I know of several cane rod guys who hunt down the old 7' 7-wt spin/fly combo cane rods and convert them to full-time fly rods specifically for redfish. I had one of these for awhile, finally broke down and sold it, but it was truly a line-blasting cannon. (I have one of these stashed away in original condition, and will get Rob Sherill to convert it for me one of these days.)
When you get to fast rods like TCR or RPLX, the length definitely adds distance, which you don't necessarily need for walking on redfish. I like the magic wand approach of a rod that accurately handles a short cast as well as a long one.
Advantage of mid-length rod in a kayak is it doesn't get in the way like a 9' rod can.
Also, if you're sitting in your kayak, you're probably going to be on top of that fish before you see him, and a shorter rod is much easier to handle and fish in close.