If you're only looking to get a lower gear- why not replace the freewheel/cassette for a 14-28 and stay 6 speed. The indexing stays the same- the RD may work.

I really only care about the range- having the steps between makes a difference when racing and to some extent, comfort. I've found I'm perfectly comfortable with a 6 speed- and actually just started using 3x5 bikes- I don't notice a whole lot of difference.