The APU in the newer OLED version is slightly more power efficient and can thus reach higher clock speeds. It shouldn't make much of a difference in max performance. But performance dips are much rarer sinc both the GPU and CPU are more likely to be able to reach high speeds at the same time.

I just got a new backplate with better cooling for my LCD Deck for much the same reason. I could increase the power limit by 2-3 watts to help make everything smoother.

Personally I don't expect a new Steam Deck to come out any time soon. I bet the next Valve hardware will be the Deckard, their new (probably standalone) VR headset. Whatever they learn from that will probably be used for a new Deck.