I think you will find that the reference to the 93+ spindles refer to upgrading the brakes, not the hub swap. I am pretty sure (but I haven't done it) that you don't need to upgrade the spindle for the hub swap.
But after suffering with the brakes on my 89, I did the spindle swap and put on Sport brakes and they are 1000x better, being able to put the Mustang PBR calipers is now possible as well (and I might do that so I can go back to being able to use my 15 inch wheels if need be).
I don't think that stock 93+ brakes are any better than pre 93 brakes, but the spindle allows you to upgrade like mentioned already. Also the older spindle the actual wear part of the brakes is part of the spindle, and on mine it was grooved and worn pretty bad, where the pads seemed to not want to smoothly travel, with the new ones it sits on a bracket and not only is it better material but replaceable.
97 Mark VIII LSC/Silver/Gray leather
97 Cougar 4.6 Sport/White Opalescent Tricoat Metallic /Gray leather down but not out?
89 Cougar LS 3.8/Oxford White/Gray velour/HG need replaced AGAIN
00 Grand Marquis LS/Vibrant White/Gray velour
63 Falcon 302/black and rust/shed isn't moving
82 F-150/Oxford White/dumb VV carb doesn't pass emissions