Hi Suziq, i have a penguin that has the axle over spring conversion done and it is good. It adds a significant amount of clearance. We have done some rough dirt roads and minor creek crossings (nothing seroiusly rough or deep though) and the only problem we have had was 4 of the 6 rivets that hold the door on broke - this happens on the off road jaycos too!
If you have an annex you will have to make the walls longer so it still they still reach the ground - otherwise, GO FOR IT!