By default FAST search sort is based on ASCII sort order. Therefore, sorting will follow [0-9A-Za-z] as pattern i.e. Numeric values will be followed by capital letter followed by small letter words in ascending sort.
In order to change the order to be case insensitive, we need to a change the fullsort profile. Index-profile contains details for each managed metadata property and sort profile needs to be changed in the same place. In the FAST installation folder inside index-profiles locate the file "deployment-ready-index-profile.xml". [ We should take back up of existing file before any changes, so that we can revert back to original settings as needed]
Add this for the relevant property: <fullsort profile="generic" />
deafult value: <field fullsort="yes" ..................... name="PropertyName" description=""/>
Modified value: <field fullsort="yes" ..................... name="PropertyName" description=""><fullsort profile="generic" /></field>
After doing a full crawl data will be presented in case-insensitive order for the given property. We can further set the order as ascending or descending as required.
This post has a similar discussion on this subject.
One of the limitation of this implementation is that once a new metadata managed property is added or an existing one gets deleted the index-profile file is re-generated. The newly generated file does not retain the modfied values.