Length of exhaust pipe

[FFTS]

I want to lift Lolas exhaust higher. Does shortening the pipe length have any effect on performance or does it HAVE to be a certain length?

 

[Smart1275GT]

I'd say it would make it slightly louder and slightly less back pressure. How much different will it be?

 

[FFTS]

6 to 12" shorter maximum!!

 

[Smart1275GT]

Shouldn't make too much difference, my only concern is the noise level at MOT time (unless a pure off road vehicle).