My universal nvidia control panel FPS cap is 140 on my current 2080 at 1440p. I'm going to keep it at 140 on 1440p when I get my 3080 Hybrid. To me, smooth gaming is substantially increasing your -minimum- fps, NOT your maximum fps. Does it mean that a more powerful card will use less heat to reach the same performance? If no, will undervolting help any with heat?
It depends on the performance you want and how much power it takes to get there, but there's only so low a GPU can go before it's not stable or able to. For example, let's say the RTX 3080 gets 100 FPS in a game and the RTX 3060 Ti gets 60 FPS. You can probably limit the power of the RTX 3080 to match the 3060 Ti and possibly get better performance. However, if the RTX 3060 Ti can still achieve 60 FPS at say 60% power, the 3080 is not getting that close because most tools limit the power of the GPU down to about 50% (120W vs 160W at this point). Though if you cap the frequency using the VF curve (discussed later), you may be able to further reduce the RTX 3080's power consumption.
I also don't think video cards power gate anything except for shader clusters if the GPU is dead idle for several minutes. So there's always a minimum handicap a more powerful card will have because they'll have more things turned on even if they're not running as hard. Like the RTX 3080 has 10 VRAM chips whereas the 3060 Ti has 8. Those VRAM chips are never power gated, so the 3080 is always going to consume more power to keep its VRAM up than the 3060 Ti.
And keep in mind, minimum FPS is dictated by the CPU first, then the GPU.
On the subject of undervolting, the way to do it with GPUs is a bit more involved than with CPUs. Tools like MSI Afterburner or EVGA Precision offer you what's called VF (voltage/frequency) curve tuning. With this, you're telling the GPU what frequency you want for a given voltage. So you can either aim for a specific voltage and cap the GPU to the highest frequency it's stable at, or aim for a specific frequency and find the minimum voltage the GPU is happy with.
Or you can just power limit the card to whatever performance works for you. I've been doing that with one of the main games I play because performance doesn't improve at all past 75%, yet the card happily consumes more power.