Your limp wrists will get used to the weight of the mouse over time.
I had a R.A.T.7 since shortly after it's launch, and just yesterday received my replacement RAT8 from Amazon.
My only real complaints about the new mouse over the old is MadCatz cheaping out on the extras with the mouse; with the R.A.T.7 you received an aluminum case that could hold all the extra parts you didn't use with your mouse, and the mouse came with a total of 5 weights that you could add or remove from the mouse.
Software is much better on the RAT8 vs. R.A.T.7, though the lighting is quite confusing. You can only select three colors to have it rotate through when in "rainbow" mode. And on that page it looks like those three colors respond to specific areas of the mouse in that case, which it does not.