Yeah, that's why I was asking. I guess you already figured out that the Tanabe subfram spacers work with the bushing to stiffen everything up, and they aren't meant to replace the bushing. I know you can buy harder rubber bushings from Nismo through Courtesy Nissan. Also, SPL Parts makes metal subframe bushings that would make the Tanabe ones unnessecary.
http://www.splparts.com/
Go here, click on S-13, then click on multi-link, and scroll to the last thing on the page. You could buy these and then sell you Tanabe spacers. Good luck.