Performance penalty of visibility info in indexes?

Has anyone actually measured the performance overhead of storing  
visibility info in indexes? I know the space overhead sounds  
daunting, but even if it doubled the size of the index in many cases  
that'd still be a huge win over having to scan the heap as well as  
the index (esp. for things like count(*)). There would also be  
overhead from having to update the old index tuple, but for the case  
of updates you're likely to need that page for the new index tuple  
anyway.

I know this wouldn't work for all cases, but ISTM there are many  
cases where it would be a win.
--
Jim Nasby                                            jim@nasby.net
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)