Would there be any point...

[Radius]

in putting, say, a 9 sp. Dura Ace rear mech on a bike with a Tiagra groupset everywhere else? Would it really make a difference?

 

[Smokin Joe]

No.

Gear change quality is controlled by the shifters, the higher end mechs are lighter, look better and probably last longer but they all work as well as each other.

 

[bonj2]

you would have the advantage that youwould be more highly regarded by those 'in the know'.

 

[Radius]

Thanks Joe,
and bonj, lol, being highly regarded by those in the know is about as useful as not having mudguards....

*exit*

 

[Steve Austin]

There would be a point in using a 105 or even an ultegra though, better mechs shift better, last longer and work better.
But if you got money to burn, then its upgrading shifters first, then mechs.

 

[Chuffy]

you would have the advantage that youwould be more highly regarded by those 'in the know'.

No.
Those in the know would suss that you'd blinged up your bike with the wrong bit. Standard practice in the trade, unless I'm very wrong. You quite often see new bikes with a rear mech one notch higher than the rest of the running gear.