One reason we can't use this setup is because it features rod ends at the inner TCA and compression strut chassis positions.
I wrote this originally, but have since changed my way of thinking. I'm going to quote my original thought just for consideration:
I think a custom TCA will only be legal in DSP if we leave the outer TCA bushing hole "pretty much" similar to stock. The inner TCA part will have to have a plastic bushing, and the ball joint end can be a spherical bearing. Of course, it can be adjustable. Now, one thing that I've wondered is if we can replace the outer TCA bushing with a spherical bearing like RatFink did. The rulebook states that we can replace the TCA with an adjustable TCA. It also states that sway bar bushings are unlimited. Well, if the outer TCA bushing is considered to be a sway bar bushing, then the RatFink mod is legal.
When it comes to divorcing the sway bar and the compression strut functionality, I worry about that a little. The rulebook states that you cannot substitute a sway bar that serves the dual-purpose of being a compression strut if doing so changes the geometry. Thus, if you install a compression strut that moves the ball joint a little forward or a little back from where it was stock, you've changed the geometry and the setup is illegal. Well, where was the ball joint stock? If it moves 0.001", will you get protested? It sounds very gray to me.
I think the A#1 question to have answered first of all is whether or not the outer TCA bushing, where the sway bar attaches to the TCA, is a sway bar bushing. If it is, then I'd like a clarification that we can install a spherical bearing there.
I think when it all boils down, we should use a car with a "normal" strut suspension for inspiration. Basically, imagine that our car, with its combined sway bar/compression strut, offers us no advantage over any other strut suspension. After all, it doesn't. Let's imagine the E46 BMW front suspension. The sway bar end attaches to a tab sticking out of the strut, and is in no way part of the lower arm. The lower arm is shaped like an "L", which is basically a clearanced triangle. Now, if we substitute a front-mount sway bar on our cars, we basically need to replace the TCA and now-missing sway bar leg with a lower arm like the BMW has. This is a solid part that connects the ball joint to the inner TCA point to the rear chassis point. Because of this, I see no reason why we can't run a solid arm that connects those same three points if we run a front-mount sway bar. This leads me to believe that not only is running a spherical bearing in the outer TCA position legal, it's stupid since we can just weld a compression strut to the stock TCA to get a solid lower arm that connects all three points.
I don't know if I'm making any sense here, but what I would offer up to the SEB is a letter seeking clarification of this:
"If we substitute the front sway bar with a front-mount design, which is clearly legal, do you agree that the XR4Ti suspension then reduces to a traditional strut suspension like a Ford Focus or an E46 BMW, where the lower arm is a one-piece unit that connects the ball joint, the lateral point, and the compression point."
Ben - I would encourage you to work closely with me on this before querying the SEB. I'm distinctly interested in this issue, and I would like to draft and extremely well-thought out and professional presentation for submission. Two brains are better than one, and I'm interested in putting a big chunk of time into this. Seeing as you called me tonight, you probably agree.