I use a 25ft extension all the time and the tangible settings like distance are still accurate.  When I need to go longer I use the Audyssey Pro kit and Audyssey Pro AVR. 

I keep a 75' XLR cable in the Audyssey Pro kit so can go 100ft using it and the included cable.

I also keep the standard Audyssey mic & 25ft extension in the Pro kit because I'll use the mic stand from the Pro kit when calibrating the lower end AVRs with the included mic.