PERF: .agg(built_in_funcs) not always calling cythonized versions

[jreback]

related #7570

agg(min/max) are NOT calling .min()/max()
https://groups.google.com/forum/#!topic/pydata/xinuPFPVakY

need to check:

sum/mean look ok

[jreback]

closed by #8155