If you chart the business transaction measures themselves rather than using those monitoring overview dashboards you can change the aggregation to other percentiles. Business transactions need to be configured to store this data (percentiles) but it is done by default and calculated hourly.
Also note there is no concept of slowest or fastest percentiles when selecting an aggregation - there is just percentiles so for instance the 95th percentile says 95% of users had response times faster than it and 5% had response times higher (i.e. the slowest 5 percent). Likewise the 5th percentile would say 5% of users had response times faster than it and 95% had response times above it (i.e. the fastest 5 percent).
I will add the measure explicitly on the "Calculate Results" section of the BT. When create your measure of interese from which you want the percentile data, make sure you tick the option "Calculate Percentil data" under the details pane:
With this config you will have the BT results with percentil details.
Hope this helps!