I promised a better explanation of my shop's findings, so here goes. The attached diagrams are from UUC's own DSSR webpage.
Both the OEM selector rod and UUC's double sheer selector rod (DSSR) serve to connect the pivot point at the bottom of the shifter with the transmission itself.
The OEM selector rod bolts up to the passenger side of the shifter pivot point and the transmission linkage via pin and bushing connections (the pin passes through the bushing and is held in place by a clip on the driver side of the two connection points).
UUC contends (probably rightfully so) that because the OEM selector rod only bolts on to one side, this setup exerts a large amount of torque, which has the potential to deform the plastic bushing in the selector joint over time, which will result in lateral play or slop in the transmission. See the gifs below:
http://www.uucmotorwerks.com/dssr/OE_SRGJ2.gif
http://www.uucmotorwerks.com/dssr/OE_300.gif
To mitigate this potential slop, UUC came up with the double sheer selector rod (DSSR), which has two prongs on either end to bolt up to both sides of the shifter pivot point and the selector joint. Keep in mind that none of us should be feeling the slop that the DSSR is designed to prevent after only 10k or so miles. The DSSR is designed to prevent the slop that occurs once the plastic selector joint bushing wears out. UUC's DSSR kit replaces the selector rod itself (of course) and also the pins and retaining clips, but does NOT replace the selector joint bushing (the OEM bushing is re-used).
The idea is a good one, but the problem occurred in UUC's execution (at least in the DSSRs that Nick and I received). One issue, which Nick and his shop isolated, is that the supplied washers with the DSSR kit aren't quite thick enough. Following Nick's lead to solve this problem eliminated about half of the lateral slop that I was experiencing, and I could actually live with the DSSR at this point if I weren't a perfectionist. The second issue, which my shop found when they took a look at it, is that the pins provided with UUC's DSSR kit are just a touch too small in diameter, which creates wiggle room between the aft pin and the selector joint bushing. Ironically, this is the very problem that the DSSR was designed to prevent. Both of these issues allowed the DSSR to slightly torque independently of the selector joint. This play is amplified by the length of the shifter, which translates into a significant amount of lateral movement at the shift knob.
The description and diagrams above debunk UUC's explanation that the lateral play that Nick and I were experiencing was due to inherent slop in the transmission. Neither Nick nor I experienced this slop before installing the DSSR, which means that the slop must be occurring at (1) the connection between the DSSR and the selector joint or (2) the connection between the DSSR and the shifter or (3) both.
The solution that I am pursuing with my shop to address this second issue is to fabricate a very slightly larger diameter pin that will fit snugly in the selector joint bushing. Consequently, this may require us to very slightly drill out the holes in the aft end of the DSSR, but we won't know that until we try it.
Obviously, I could go back to stock, but my bushings would eventually suffer the wear that the DSSR was designed to prevent. As I plan to keep this car for some time, I'd like to avoid that situation. Sure, I could always replace the plastic bushings when they wear out, but that's treating the symptom, not curing the disease. Another nice feature of the DSSR is that it's heavier than the stock selector rod, which noticeably smooths out some of the notchiness of the shifts, similar to a weighted shift knob.
I hope this post helps to clear up any confusion regarding the issues with the DSSR. For those of you waiting on UUC's purported fix, if it ever comes, I wouldn't be satisfied unless it addresses both issues described in this post. I'll keep everyone posted on my progress.