Spined Threshers are a harder fix. It's not an upgrade that makes them good, it's the base stats.
For the cost, they hit to hard, hit to soon, get a built in bane/re-roll generation, and can take a hit like nothing else.
When they first came out, my regular opponent didn't think they were "that bad". So I ran nothing but threshers in a few games. It was ugly.
Ran 4 block of 4 one game, 11 singles the next. A 6, 4, 2, 2 and 1 in the final game. It didn't matter how I configured the units, it was hideous.
The 11 singles was the most offensive; 22 dice red dice with 11 re-rolls; damage x2. They chewed through everything. Everything is a single model, you can always ignore your whole army for collisions when moving (as long as you clear).
IMO, the best fix is Army Comp. Limit either points or trays spent on Siege units. No other faction has such in imbalance on siege that they would want to spam these like Uthuk does.