As we made a lot of changes we could have a closer look at what the browser support status is.

Oscar from GPNL showed me a data-based config from real GP usage statistics:

• .browserslistrc
• The stats are here: browserslist-stats.json

For the rest of the GPNL check:

• .babelrc

There is a limitation to data: if IE10 users can't use the site, you won't get any % of iE10 users. 
So I would say that if there is an minimum-effort (and low impact on performance of all browsers) way of extending the browser support, go for it (e.g.: adding some polyfills, but not too big ones).