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V.34 has a collection of implementation options which will affect the 
eventual performance of the modem.  Some options to look for in a product
are asymmetrical transmission, auxiliary channel, lower speed baud rates,
nonlinear encoding, precoding, trellis states, etc.  Good quality modems
will have implemented more of the V.34 options.  [Hopefully they would have
also done a *GOOD* job in the implementation.]  

Data Communicatons magazine, Nov 1994, pg 99-108 is a good reference to these
options and their impact on interoperativity and performance.

Two good V.34 modems are more likely outperform two Joe Average modems.  The
enhanced performance will be noticeable on lousy lines, and may be noticeable
on good lines.

This also means that a good V.34 modem connecting with the "Joe" modem will
not notice any performance benefits.