I have 12-34T cog with Shimano 4700-GS rear derailleur. The bottom of the cage is almost in line with the outer edge of the rim when fully extended, so all that's keeping it from touching the ground is the inflated tire height, which is acceptable for me. I can't remember the measured length of the cage. Different models will probably have slightly different lengths even if they're all GS.

I then have a front derailleur with double chainring with 24/44T. It gives me 519% range. The front derailleur was attached to a braze on adapter that I made myself.