Firstly, if you are going to do the cruising you suggest, I would certainly go with the diesel-only or hybrid configuration and not the all-electric configuration. In part because the faster you go with the fins, the better they do their stabilizing job. At least this has been my experience on the GL33.
At anchor, my GL33 with fins can be quite uncomfortable with rocking. I bought some Davis flopper stoppers and they help, reducing the roll by ~20% but they are a real nuisance and take a lot of room to store. I long for something like a SeaKeeper.
I have 4xZipWake 450's on my GL33, which I installed to get the bow down on the rare occasions when I run over 7 1/2 knots. They work well for this. And at the high end of the speed range, the ZipWakes result in an approximate 15% fuel savings for the same speed. BUT, if your interest is primarily stability, I would not recommend ZipWake. In beam-seas, I can notice a difference with them on vs. off. But it's only a 5-10% improvement.
I would seriously look into a SeaKeeper instead -- if you have the space, the energy, the weight capacity and the budget. I was very interested in a SeaKeeper, but quickly came to realize that the size and weight would be impractical for my GL33. It might be feasible for a GL40, however.
As far as the small black water capacity - that's a minor thing. Don't let that stop you. It's easily addressed by plumbing in an extra tank. My GL33 came with a 63L tank and I had an additional tank of 67L custom built to fit an oddly-shaped space in my bilge. Alternative solutions abound for this, so it's not a show-stopper for your purchase.
