I ran a G6 on my 400 gallon quite successfully for about 2 years. Mine had 3 Sedra 9000s which gave me no trouble during that period. The cone leaked where it joins the main body, so I eventually just sealed it permanently on there.
I don't think you are as oversized as you think. Eventually, I had to replace my G6 with a significantly larger skimmer. (I doubled the chamber size) That replacement seems to have solved some major problems I began to have as the system matured.
For the money, I think these are a good deal. Eventually, you will replace it...but that is half the fun of this hobby!