Mauer has a good chance to surpass Puckett this year in career WAR:
Assuming he ends the year at around 45 WAR, he would be 21.3 WAR behind Harmon for the all-time leader.
If Mauer plays out his 4 remaining contract years and 3 twilight DH years (retiring as a 38 year old), he would have to average around 3 WAR per season to surpass Harmon Killabrew. Those numbers are well below his WAR average from 2006-2013, but an aging DH/1B may struggle to continue that kind of production.
My guess is that he ends his career with around 60 WAR, second all-time for the Twins behind Harmon and ahead of Rod Carew.