One disadvantage when using an RB25 head - and one of the ones that dissuaded me when I went for a rebuild recently - is clearance.
AFAIK you need to either put a front facing plenum on or drop the sub frame. The first I couldn't afford or find, the second sounded dodgy to me.
If I ever have to rebuild again I will use an RB26 head on the RB30 bottom end though.