First of all, Go's strings are also immutable. To get around this, you can use the []byte type, as you said, but Go is built with the assumption that all text is UTF-8. Frankly, I find the suggestion that ASCII is good enough, even for Americans, absurd.
Second, Python's "list" type and Go's slice type (what you're calling a "true array") are almost the same under the hood. Both are variable length arrays. (I'm not sure why Python calls it a "list" since it's not a linked list.) The only major difference is that Python's lists are always filled with pointers to the underlying objects. In Go, you have the choice of whether you want to use pointers to the underlying objects or to put the objects into a contiguous block of memory.
Speaking more broadly, there's no reason you couldn't use a lot of different Go practices in Python and end up with much more efficient Python code: pre-allocating lists of the proper size, for example, or making sure to use mutable buffers instead of immutable str objects. However, Go makes it natural to be concerned with these efficiencies, and Python makes it natural to ignore them. Each language has its strengths.
g = bytearray('Do this')
g
>>> bytearray(b'Do this')
g[:2] = 'On'
g
>>> bytearray(b'On this')
Meanwhile, since Go has true arrays, and since you can probably just assume we're in English only and let byte[] substitute for real unicode, you can write very nearly what you'd write in C, albeit more safely, and not be creating string objects by the thousands, and it wouldn't be particularly unnatural Go code. A 12x differential is very plausible. I'd believe you could even go higher.
This is not to say that Go is awesome or that (C)Python is a bad language in general. I like (C)Python. But it does have some bad cases in it if you need lots of performance, fiddly little string algorithms and lots of numeric computation being two of the big ones. (By which I mean numeric computation in pure CPython, not with Numeric Python or PyPy.) In both cases the sheer rate of object creation and destruction work dwarfs the real work being done.