Last year Mauer hit .324/.404/.476 in 113 games: 75 at catcher, 29 at DH, and 8 at first. Let's assume he does similar in 2014 but with 150 games all at first.
DRS rates Mauer's defense over a full year (unsure if that's 150 or 162 games) at +2 runs; UZR says +8 runs; TZL says +7 runs. I'm going to assume +5 runs.
Extrapolating things a bit to more games, but at first, gives the following rWAR figures.
| Year | G | PA | Rbat | Rbaser | Rdp | Rfield | Rpos | RAA | WAA | Rrep | RAR | WAR |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2013 | 113 | 508 | 26 | -1 | 1 | 6 | 2 | 34 | 3.8 | 17 | 51 | 5.4 |
| 2014 | 150 | 654 | 34 | -1 | 0 | 5 | -12 | 26 | 2.9 | 22 | 48 | 5.3 |
Almost exactly the same. A large part of that is due to many more plate appearances. I don't know how much his defense will improve at first given experience. DRS really liked him this year, but UZR didn't. If instead he averages +10 runs at first, that's bordering on a six-win season.