Those extra transistors don't get used for free. Zen3 is a larger core than Zen 2 from a transistor count perspective. Those extra transistors aren't there as window dressing, but to be used. Zen3, as it exists on Cezanne, is still using essentially the same process node as Renoir did, so, the only advantage that it would have with respect to power draw would be general layout efficiency improvements in the Zen3 core, which we don't know really exist, and improvements in the power management and voltage distribution design of the processor itself. However, when compared to Lucienne, according to AMD, it also has those same power management improvements, which means that that advantage won't even be there.
I'm actually quite interested in the comparison between Lucienne and Cezanne in the U series lineup. The most telling comparison will be between the 5700u (essentially an improved 4800u) and the 5800u. I have a feeling that the 5700u will be just as fast, or even faster, in a small set of MT tasks, especially ones that scale well and don't have a lot of communication between the various threads and aren't highly L3 cache bound. It'll likely also be just about as fast for games that are lightly threaded, especially when using the iGPU.