Ya know, I'm not 100% which one I have in the car. I own four of them, two in my truggy, one in my SCT and one in the Ofna... ha ha.
Either the 56 or 58 will work. But yeah, the 58 is all you need. Burt even with half the response time of the 58, the 56 will still feel plenty fast, and more than enough for an 1/8 GT car. In fact, a little slower servo will prolly make the car easier to drive, on the track and for speed runs... Again, less twitchy.