Neil, check the box ratio's of the R & D first to make sure your happy with it - if you do decide to go that route.

i came across a few a couple of months back but sixth gear was a ratio of 1:1, which is the same as your standard 4th!!

there are a few other reasons aswell with the R & D box which has made me go for the Mullet/Reyland/Barra box.