UL amps less powerful?

[Chuck H]

FWIW and not yet mentioned...

UL was originally designed as effort to get more "triode" like distortion performance with a more "pentode" like output. A best of both worlds intention. It's worth noting then that operating pentodes wired to operate as triodes always means lower wattage. In other words and contrary to some modern misunderstandings, UL is not and was not ever considered a way to get more power.

[pdf64]

I recall a thread or 2 here some years ago, perhaps it was potatofarmer that put forward the hypothesis that the screen grid taps with those Fenders was to take some stress off the screen grids whilst the anodes were on a stiff 500V supply.
With a resistive load, instantaneous max screen grid dissipation occurs at peak anode current, because the screen grid voltage is very much higher than the anode voltage.
A tap off the OT primary feeding the screen grid reduces that differential, but with the downside that peak anode current will be reduced.
The designer's task was to balance the screen grid tap point such that audio power output was maximised without screen grid dissipation being excessive.

[Helmholtz]

...., but with the downside that peak anode current will be reduced.

...which is the same as saying Vsat is increased.
Peak current and Vsat are inversely related.