On short GOP lengths it's ok ... as in your case you're happy with.
KVCD = max GOP length = 24 ... thats not that very much (in case of a OneCD encoding template, and thats also why you don't see a big difference BUT!: This also depends on the Bitrate you are using to encode and many other factors. So it can't be determined in general.
But on longer GOPs as this means more far far away "bi-directional" references to the previous/next I frame ... the quality at the frames within the half of the GOP could suffer.

Imagine that there will be a reference to the next I frame too.
Also explained here:
http://www.broadcastpapers.com/sigdi...-%20comp09.htm