I ran http://hg.python.org/benchmarks/

Result:
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$ python3 perf.py -r -b default ~/prog/python/default/python.orig  ~/prog/python/default/python.patched 
INFO:root:Automatically selected timer: perf_counter
(...)
Report on Linux smithers 3.18.3-201.fc21.x86_64 #1 SMP Mon Jan 19 15:59:31 UTC 2015 x86_64 x86_64
Total CPU cores: 8

### etree_parse ###
Min: 0.273882 -> 0.297294: 1.09x slower
Avg: 0.277947 -> 0.312787: 1.13x slower
Significant (t=-11.54)
Stddev: 0.00446 -> 0.02987: 6.6906x larger

### fastunpickle ###
Min: 0.510721 -> 0.526520: 1.03x slower
Avg: 0.521282 -> 0.542610: 1.04x slower
Significant (t=-8.33)
Stddev: 0.01024 -> 0.02348: 2.2931x larger

The following not significant results are hidden, use -v to show them:
2to3, django_v2, etree_generate, etree_iterparse, etree_process, fastpickle, json_dump_v2, json_load, nbody, regex_v8, tornado_http.
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I ran again etree_parse alone and the slowdown disappeared :-p
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The following not significant results are hidden, use -v to show them:
etree_parse.
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Output when I ran fastunpickle alone:
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### fastunpickle ###
Min: 0.510994 -> 0.522419: 1.02x slower
Avg: 0.524633 -> 0.536876: 1.02x slower
Significant (t=-2.69)
Stddev: 0.02983 -> 0.03436: 1.1518x larger
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2% slower on a single test doesn't look to be significant. Raising a SystemError is more important than the minor slowdown. What do you think?