I suppose that the 70 MPH rule is the most widely prescribed as a point to upgrade.
I did mine last year concurrently with a motor upgrade, having alot of faith in the mods...
I can tell you this though, There was a noticable difference in the steering feel after the upgrade. prior to the change, the steering whell feel was fairly light, kind of easy to turn in either direction. there was a small amount of play in the wheel also, which made the boat kind of hunt side to side in no wake zones.
After the upgrade the wheel was tighter, it took a little more effort to turn but it now feels much more responsive. Great "road feel" if you will.
The low speed hunting is gone, the boat goes where it is pointed.
Worth the money.
Besides, all that stainless hanging on the transom looks really cool