Upcoming APUs sporting Zen 4 cores and RDNA 3+ graphics are claimed to be much faster than existing chips, and look ideal for handhelds.
Honestly I think it's probably too soon for the kind of significant performance increase Valve is wanting for the Steam Deck 2. Not to mention that the OLED deck just came out. That said, even if these chips don't make it to a Deck 2 I'm sure we'll see them in competing devices.
The relevant metric is how much faster it is at the same power draw. The Z1 Extreme/7840U/8840U are faster compared to the Deck's APU, but at equal power limits there isn't too much in it.