Language implementations that share a compile target have the same lower bound for performance (being fast) but they have infinite and unrelated upper bounds for performance (being slow).

Let's say there's a Python to C compiler (there is) and let's say there's a C++ to C compiler (there was, may still be idk). The performance characteristics of programs compiled via both won't be very similar at all. They can't be any faster than C. They can both be much slower than it. They'll differ wildly from each other.

No, they don't perform the same, just like different languages that compile to native machine code don't all perform the same. It depends a lot on the compiler, which depends on both the language and other goals like compilation speed, memory/storage usage, effort making the compiler, etc.

Wasm doesn't yet have a GC. So, when you compile languages requiring GC like python or Go, they have to include that GC in the resulting wasm binary, their wasm GC implementation might not be as performant as native either. On the other hand languages like C or rust doesn't have this overhead. This makes wasm binaries from GC languages big in size and will affect their performance.