A generous member of my bike co-op gave me a used Ultegra RS 6500 rear derailer, knowing I want to upgrade from a Sora on my Soma Double Cross (with friction bar ends). My research suggests that the 6500 (long cage) merely pulls to a max 27t. Is this right? I'm using a 12t-30t 8-speed Shimano cassette and would love to slap on the Ultegra but not if it won't properly reach the bigger rings. If that's the case, can you suggest a better real derailer upgrade? Thanks.

It depends on the particular bike. It may work well or it may require lengthening the "B" screw. The maximum cog size limit is not a function of cage length.