I do feel like the possibly lighter hitting outfield is somewhat ablated by our double play tandem having run an .870 OPS last season...
But further, if you look back through the Mariner's pitching stats as a team, you might notice 4 years that stand out: 2001, 2003, 2009, and 2014 are all seasons where the Mariners pitchers beat their FIP by more than 10%, and each year except 2014 was a year in which they used a 3 CF Outfield.
2001: 3.54 ERA / 4.12 FIP - Outfield of Cameron, Ichiro, and a platoon of Al Martin & Stan Javier (who had 6,000 CF innings between them, playing at the end of their careers)
2003: 3.76 ERA / 4.29 FIP - Outfield of Cameron, Ichiro, and Winn - Outfield UZR: +49.5 / DRS: +32
2009: 3.87 ERA / 4.39 FIP - Outfield of Gutierrez, Ichiro, and (planned as) Endy Chavez in left (we were questioning the 3 CF paradigm that year too, until Endy's knee exploded in a collision), followed by Ryan Langerhans, Michael Saunders, and Wlad Balentien - Outfield UZR: +62.4 / DRS: +48
2014: 3.17 ERA / 3.61 FIP - Outfield of Austin Jackson (until his trade, when this became the team WITHOUT a true CF), Dustin Ackley, Michael Saunders, James Jones, Endy Chavez, and Stephen Romero - Outfield UZR: -4.2 / DRS: 2 (Honestly I'm shocked it's that high). 2014's FIP beaters were lead by Felix Hernandez (2.14/2.56), Chris Young (3.65/5.02), Tom Wilhelmsen (2.27/3.74), and Joe Beimel (2.20/4.18), while most of the other pitchers were pretty close to their FIPs, so I'll chalk that season up to fluke.
The other seasons, however, show a much more equal distribution of ERA gains vs. FIP. So the CF hat trick has kinda been tried a few times in Seattle. The trick seems to be about having the offense to go with it. The 2001 M's had their top 5 hitters post a 120+ OPS+, and their top 11 batters sported OPS+ of 89 and above. The 2016 Mariners traded or let go 7 of their 11 top hitters from last season, but the 4 they kept fit the template, so I guess we'll see. Gotta say though, looking back at the starting pitching in 2001 is a lot like looking at this season WITHOUT James Paxton loaded and ready to go.