First off, all versions of this patch do away with the rather aggressively repeated inline code.  My previous comment about refactoring and testing an inlined form were purely an experiment with null results.

That aside, you do raise a good question.  However, given the current patch, it is unfortunately off-topic and irrelevant to the consideration of this patch.  Please feel free to pursue it elsewhere, since I worry that it will only serve as a negative distraction from the much more interesting aims of the mro optimization.  Since you are clearly worried about the performance of attribute lookup, please try it out and report your findings.  I'll be happy review the results from your benchmarks and any suggestions you have.