My red clay gest so hard that my full sized backhoe has trouble breaking through it in the heat of summer. Here, the clay is like a pie crust. It's hard at the surface, but gets softer the farther down you go, but mostly, you just have to break through that top layer of hard baked clay to get down to the easy digging stuff.
I started out with a cheap Leinbach line post hole digger and it was horrible. I used every type of leverage out there to get it to dig, and it still struggled. I switched to a Speeco with replaceable cutting teeth and it was a lot better. When I couldn't find any more teeth for the Speeco, I bought a Countryline auger from Tractor Supply. This was out of desperation since I thought Countryline was a junk product.
I was wrong, that auger digs through the hard clay like it's not there. It digs TOO FAST and my biggest challenge is slowing it down so it doesn't burry itself!!!!
Before spending that much money for a kit to push your auger down, I would try the Countryline auger first. For me, it fit on my Leinback Line gear box, so it was just two bolts to make the switch.