Would the Acratech GP be a good choice, do you think? And if so, where would I best go for my L-plate?
You're welcome, glad to hear all our ramblings are of use.
The Acratech GP should be able to support your lenses and should be reasonably durable. Whether it is best for your style of photography only you can really say. Part of using gear is liking the feel of it, feeling like it is an extension of yourself. I appreciate the texture of the RRS clamp knobs, the fast closing action on some Kirk clamps, the smooth control of the BH-55 head, the huge variety and mounting options of the Hejnar macro rails, and how much easier life is with ring mounts on telephoto and macro lenses. If you're like most of us, you'll have your own likes and dislikes after using something heavily for years, and you'll appreciate minute improvements in the newer gear. Then maybe you'll try something else. Camera gear is not "...'til death do us part," after all.
Note that you might be able to order an Acratech head without a clamp, meaning you could buy the clamp of your choice from somewhere else. For example, a RRS panning clamp might be more practical than their "upside down" posture for frequent pano-stitching. The clamp-free versions used to be common from Acratech--contact the company via email if that's an option you'd like and see what they will do pricing-wise. It never hurts to ask. I notice they now have lever clamps and bubble levels available, so maybe this option doesn't matter much to you.
Both Kirk and RRS make great L-plates for camera bodies. Usually I check weight and whether I have full access to the controls, but Kirk and RRS are both good about making ports available. Sometimes Kirk is a little cheaper. With respect to your 7D, however, the prices are the same, so I personally would choose the RRS since it is a half ounce lighter (unless you need the new sensor center line on the Kirk).
Hejnar really doesn't make L-plates. They're costly to design, as designers must buy all the camera models and accessories to test out prototypes. Hejnar carries a couple for a few odd Nikon and Sony models, but doesn't appear to customize them well to the camera bodies. I think he mainly fills a niche in L-plates for camera bodies that the bigger companies don't want to bother with.