The GS documentation I've seen says it simply prevents your macros from getting stuck on an ability. Which is basically the same thing as putting your abilities on a single key in is boxer. My DPS meters show the same thing yours do: effectively no difference. For people that don't use isboxer, I'm sure it's a huge boon. For us, I dont see much benefit outside of the GS macro postings being a good resource for ability priority for use in GS or isboxer or whatever your choice of tool is.