RMU is a long ways off, it seems, but Nicholas could better answer that. But it would be interesting to know how many SW people use the RMSS stats? I write the stats in RM2/Classic and we have someone convert them.
As for 'new' There is new art and a lot of new text in Jaiman. Haalkitaine has a lot of revised text, tons of new material, Tombs and sewers, and some gorgeous portrait art from someone new (and old but his new work is gorgeous!).