15.4:1? This would almost have to be the output force to input ratio. What is seem like you want is less turns from stop to stop, that is the turning ratio. Other than that 15.4:1 as a turning ratio would mean you would turn the steering wheel 15.4 times from stop to stop, and in that respect I guess you would want a lesser ratio....
A Z3 rack is about 3 turns from stop to stop. Some slight modifications are needed depending on the year.
On the other hand, I have heard of autocross guys using a LHD Talbot Sunbeam rack (good luck finding one), it can be fitted upside down (with some modification), it's about 2.25 turns from stop to stop.