David..Other than the RV12, the Only other RV model that has a stall speed less than 45kts is the RV9/9A.
That is why a number of members on here have built this model and currently flying to a MTOW of 600kgs.
My RV9A is GA registered and I wont hold my breath for a weight increase any time soon.
I think with any weight increase will see some strict maintenance requirements.
Mike