I haven't tried it on Deck, but personally I'm a bit skeptical you'd get much tangible benefit over just setting lower package TDP limits as necessary, considering how much of a pain in the ass stable undervolting can be. TDP also has the advantage of per-game settings so you can just crank down undemanding games automatically instead of trying to juggle voltage/stability across the board.

On my LCD deck I ran -30 on all 3, and it worked well. I originally ran -35 but Like A Dragon cutscenes would sometimes freeze. Didn't notice any other stability issues.

I have an OLED deck now, only have it set to -10, but I'm going to slowly increase it.

This article is from back before valve added undervolt settings to the default bios (and before a CMOS reset would reset the undervolt, which makes undervolting much safer now), but it still has some really informative tests of the benefits of undervolting. Less thermal throttling improves performance in some games, and he was able to get about 16% increase in battery life.

I just started undervolting the other day. Currently -30 across the board on my OLED with no negative results so far (even though I'm playing Baldur's Gate right now). I haven't yet tried to get more aggressive with it.

I think the general consensus is that it's possible to get up to a roughly 3-4% improvement in performance depending on how low you can get it. I think the battery benefit is in that same percentage ballpark, too, but I'm less sure on that one.

I undervolted my Deck at the end of last year to -40mV on cpu, gpu and SoC. Until now everything is stable and runs without problems.

In terms of performances I didn't noticed an improvement in fps but the fan doesn't ramp up as often.