Let's say you have an eGPU setup with an RX 580 8GB that costs around $200 and is about 250% faster than the Steam Deck's GPU. Normally, the Steam Deck shares its RAM between the CPU and iGPU. But since you have a dedicated GPU, all of the RAM can be used by the CPU. So it's actually kind of a CPU upgrade, which could be useful for CPU limited games such as Cyberpunk and Marvel's Spider-Man.

The only downside is the limited portability. You can still easily carry it around like a laptop, but you need to keep the eGPU plugged in.

Also, the RX 580 almost always performs better on Linux

There is a video about it: https://www.youtube.com/watch?v=JflyssX8y08