I have taken the plunge and flashed 4.0.4 onto my scroll extreme.
Both suggsy's RC1 and CM9 work well but I have decided to stick with RC1. This is because RC1 is more responsive in some situations.
A simple test is to load tablet mahjong from the play store. When clicking on a tile using the stock ROM and CM9 there is a 1second delay between touching the tile and it being selected. With RC1there is no such delay.
Thank you suggsy for a job well done.