Well I have given this one a little bit of thought and yes went out and pulled up a seat and looked at the bike for a bit....
and IMHO this is a good thing for a street bike and I will have one.... now the only question is does Mik have a source or do I need to get it fab'ed myself.
Why do I think it will be a plus for a street bike..... well again,IMHO, this is a step below bracing the frame (which would eliminate the need for this bit) and for bikes with a braced swinger and a good front end this should help out.
If you are racing or doing die hard track work, then brace the frame but for most street riding (or in my book under a ton through the bends) I have found the frame behave rather well but does "twitch" a bit if you start pushing hard.
Now back to this bit, what I see it doing is acting the same as a fork brace on a standard front end.
What I mean is that while it doesn't add much stiffness in and of itself, it does tie the two sides together which does make a big improvement.
Where does the load come from, you might ask?......Well from you the rider.
The more aggressive you ride, the more you load the sub-frame.
Even if you are only 10 stone, it can be a big load as you need to add in the length of the lever back to you.
Again IMHO, like a fork brace, if you ride aggressive you should see a difference.
If you don't push, then I doubt you would ever notice a difference.
Though I do have a question..... in the pic of the original part there are a pair of collars on the mounting bolts.
Are these to take up the space of the sub-frame if you are not running the stock unit?
I could be just full of you know but it looks like a good thing to me and like I said I will track one down or fab one sometime soon

Loud pipes don't save lives, knowing how to ride your bike will save your life.