The good news is that Stormy Mondays are a lovely clean and flexible pickup. One of life's 'safe bets'. You are of course quite right to confer with T.F and Tim, but with B.K.Ps, there is so much quality and flexibilty in them, that you will have an overlap of functionality between types anyway. I also like the idea of calibrated / matching pairs, for they surely recieved the highest design condideration ( to work as a functioning pair ) at the offset.
I was interested in the concept of the Riff Raff, as Twinfan once told me that they are sweeter sounding than one might expect from a crisper brasher P.A.F. I understand them to have a narrower range of 'voice' than the Stormy Monday's though - so if you are investing in B.K.Ps, I thought that S.Ms would be the even more transparent version of the Mule AND perhaps the warmer version of the Riff Raff. A real winner all round. I have had the S.Ms in an S.G Standard and a Les Paul. In each guitar I heard the guitar's own characteristics - not the pickup's own voice dominating.
Not that I think a 'dominant' pickup is wrong either. Even as a predominantly 'clean' player I would love to have a Les Paul with darker weighty Crawlers, or lovely upper mids rich Emeralds too !
That is the main reason I don't think ( beyond marketing hype - which Tim doesn't need to do ) we don't really need more types of B.K.P. It would take a lifetime to discover the uses for the existing range.
For example, I have had Mules and Stormy Mondays in the same Les Paul, and whilst I have heard some difference in the mids, I never thought that one type where wrong or right, just two different types of very nice !
Enjoy the experience of real B.K.P tone Robbo, (re) searching is part of the fun !
:D