Where do cables upgrades have the most impact?

The one problem with "Shorter is better" is down the road, when you move stuff around, or change racks, or buy a different gizmo. Suddenly your once just right interconnect is discovered to be three inches too short??? (the old box RCA were on the left, now they are far over on the right...)  I long ago used to believe in the just long enough IC. And over the years I have gone the total opposite. NOW, all my newer IC are 1.5 meters long. Long enough I am not going to have to worry about 'TOO SHORT $&@!!$(# !" anymore. And those are $1,100 a pop IC.                           
Just a shout out for buying a definitely longer than just enough when you buy big money cabling. For good reason, a little LONGER is, in the long run (pun intended), better. (primarily for your wallet, and I being a spendthrift cheapskate, watch that carefully)     
On the other hand, if you buy cheap cables, it hardly matters, Or if you change cabling like some folks change clothes. no problem.

Is it really the cable industry that needs our excess money the most?

The ’industry’ exists because someone who BUYS cables wanted something better. One ’better’ cable at a time... It grew. If no audiophile bought them, the industry would not exist. One actual plus for consumers is cables generally do not wear out. I still have cables that I made in the 1980’s. So aside from wanting better... no need to ever buy more. Cable company advertising is only to lure you away from SOME OTHER CABLE COMPANY. No one walking down the street accidentally saw a cable company advert and decided Gee I must NEED those bad, and rushed to buy some.
I am probably like a lot of audiophiles in having a pile of cables I do not use anymore. All told between IC, PC, speaker plus video or digital I must own thirty cables not in use, sitting in the closet. (anyone need a Monster S-VHS cable??? I only keep one because.. never know I might suddenly want to use it.. for????)   
As for "the most".. Priorities. Sometimes you might be best off spending your money on some other audio doodad. But sooner or later you might, maybe, decide to spend that $3000 sitting around on a cable? Who knows.. I did. (and glad I did)

rather than buying 4 new power cords at say, $400 each, buy a power conditioner/surge protector that delivers stable voltage and current for around $1000. I'm in Australia and purchased a locally made unit, the Thor PS10 which comes with a first class power board as well that has both coax and CAT cable protection. I went down that road, thinking more of equipment protection due to a previous issue, but I am pleasantly surprised by the sonic improvement to my system in general.

I put in some interconnects that's made my system a joy to listen to.  I can't tell you which wire will do that to your system if any but good luck.