Welcome!
I have Riff Raffs in an SG, and yes they are quite "Angus" when you want them to be. Then again, I'm in a DC band so they suit me! Having said that, a mate of mine plays blues mainly and he got a superb tone out of my SG that wasn't in the least bit "Angus"!
I have a Mule arriving this week to go into one of my Gordon Smith's, so I'll be able to compare the two at the weekend. My initial idea is much the same as yours - the Mule will be a touch more rounded, smooth and with a bit more midrange focus. That's what I'm hoping for anyway! The brazilian cedar of the GS isn't as thick sounding as mahogany, so I think the Mule will work well.
I believe Tim recommends the Riff Raff for SGs over the Mule as all that mahogany can get a bit thick - the spikier Riff Raff gives it a bit more of a punch. I'm sure I've read somewhere that Tim has a "special SG wind" for Mules, but I think that may just be a regular Riff Raff.
I don't think anyone's ever tried a Black Dog in an SG, and it will sound different to how it does in a Les Paul. It could be worth asking Tim his opinion via email or over the phone?
What kind of tone are you after, compared to the stcok Gibbo pups? More bite? More mid?
For a good all round tone in an SG, I'd go with a set of Riff Raffs.