There are parts of the zip kit that are worth it, and then there are parts in the transgo kit that aren't in the zip kit that are needed. Both fix some inherent problems to the 5r series trans.
I installed the transgo kit and then some of the sonnax parts in mine. (Most notably the o-ringed end plugs to prevent pressure loss) I'll have to look and see what other sonnax parts I used with the transgo kit.
Here's the problem if you want firmer shifts. It is good in theory BUT you can install the heaviest springs in the shift kit for hard shifts but the transmission is computer controlled with adaptive shift control, meaning the computer adjusts the pressure and shift points to keep the shifts smooth. Once you install the kit the computer simply overrides what you've done by adjusting the pressure to bring it back to shifting as it was stock. If you clear the adaptive shift memory it will shift like a rocket for about 50 miles before the computer re-learns. The only way to keep the shifts firm after installing the kit is to delete the entire adaptive shift program which I have not been able to figure out how to do. I don't know if something like an SCT tuner is capable of this.
It was still worth it for me even though it doesn't actually shift any firmer, it still feels "crisper" but my valve body was having issues before I did it. The one thing you absolutely must do without question if you do this is DO NOT USE THE VALVE BODY GASKETS IN THE KIT!!!! Go to Ford and get the steel bonded one piece separator plate gasket! The 3 piece gasket/separator plate are known to blow out and cause pressure leaks. The one piece bonded gasket/separator plate is nearly bulletproof.
As for whether it's worth it on a tranny with 140k, yes. Even if you have to rebuild the tranny down the road you still have a nice fresh valve body to use so it's never a waste.
If you are in there, you might as well replace the EPC solenoid while your at it (it's recommended by both the sonnax kit instructions as well as the transgo kit instructions,), if not the whole set. And don't buy China garbage solenoids from eBay.