I purchased a set of these Rizoma's; however, I don't know if they have a greater mass:
http://www.pjsparts.com/catalog/prod...roducts_id=178
They are a little larger than stock; but, they are billet aluminum. There appears to be an even larger set available.
If you are interested adding mass for some reason, I would experiment with adding lead fishing weights to determine if it is the solution to what you are looking for.
I did this as a means of reducing vibration in an '82 CB900F, since it I bootstrapped the whole ride and it was cheaper than fixing whatever was causing the vibration in the first place.
Now that I think of it, I don't recall if it worked.