Specs say "up to 70km" of additional solar range per day; which I'm assuming is perfect alignment of the sun with the panels for the whole day. So realistically it'd be 10km (or 3%ish of the total plugin range) if its a cloudless day and you park in full sun (this is a wild guess but seems realistic).
If you ask me, its not a huge boost, but its more than I would have expected.