You probably won't shorten the turning radius. The reason is there just isn't sufficient room to cut the wheels further. That was one limit of the K based chassis that was fixed in later cars like the Neons. Fast ratio will get you to those limits quicker but won't turn the wheels further.
Even the Spirit likely offered a base and a faster ratio in the racks. The faster ratio was likely available on turbo cars or as part of a suspension package. The other thing to watch is often the faster rack is used with larger tires. If you consider a car like the Daytona that used 14", 15" and 16" wheels, you'll hit the turning stops faster on a rack for 16" wheels than one for 14" wheels (because the 16" tires will hit the body structure faster) which means the 16" wheeled car will have a larger turning radius.
I see 2 racks available for the 1989 AA body. One for 14" wheels and one for 15" wheels. Regulated pump assist valving may also change the steering feel. Firm feel was sales code SSB. There was no speed proportional steering option offered.
See page #169 for part # R0400216 here: http://www.moparpartsworldwide.com/moparreman.pdf
As stated, the turning radius is determined by steering/suspension geometry and can't be changed.