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I don't understand this issue: can you write a short script to test a collision? 
"E.g this strings collide for every prefix ending on 0xcd"
Do you mean that prefix & 0xff == 0xcd?
"0x27fd5a18, 0x26fe78fa"
Is it a byte string or an Unicode string? b'\x27\xfd\x5a\x18' and b'\x26\xfe\x78\xfa'?
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Using PYTHONHASHSEED environment variable, it's easy to find two values generating the same _Py_HashSecret. Just one example:
PYTHONHASHSEED=3035016679:
* _Py_HashSecret = {0xcd5192eff3fd4d58, 0x3926b1431b200720}
PYTHONHASHSEED=4108758503:
* _Py_HashSecret = {0xcd5192eff3fd4d58, 0x3926b1431b200720}
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I wrote find_hash_collision.py to try to compute a collision, but the programs fail with:
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Fail to generate a new seed!
# seeds = 65298
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So it fails to generate a new random seed after testing 65298 different seeds. I ran the script with a function generating a seed, a seed generate a prefix "ending with 0xDC".
See attached program: it generates a random seed. Uncomment "seed = generate_seed_0xCD()" if the prefix must ends with 0xCD byte.