@taha13 refer to one of my older answers. Although this is for Column Chart the same can also be applied for Bar Chart to add a % after the values displayed in the chart.
1) A lot of Simple XML features will stop working.
2) HTML dashboard can not be reverted back to XML
3) Ease of Dashboard editing though UI will be lost
4) Editing HTML dashboard requires knowledge of Splunk JS and HTML, CSS, JS.
Since the feature is not available out of the box with Highcharts implementation, other option would be to use Custom Visualization API from scratch and build visualization using Highcharts. (Which would be even more time taking process)
I am sorry I am not aware of a built in option. Let me convert my answer to comment so that others can provide their input.