Nigel,
i think you can get better results using OPEN GOPs. Whichever you set (closed or open) you got only one “I” frame (a full original frame) per GOP. But in the case of the OPEN, the “I” frame of the next GOP is also used to buid the in-between frame. This is a big advantage.
Any way you should have a look to the page 138 to 142 of the Compressor manual. All the GOP structure is quite well explained. Them few tests.
Cheers,
Rafael