Has the fact he has thrown an insane amount of innings been mentioned?
1315 to Darvish's 1246 in seven seasons.
4 seamer with a little run

Sinker

Splitter

Slider

Everything comes out of the same arm slot:

He has a slow curve and a change, but doesn't throw them nearly as much as those four.
He clearly has no problem with control and command. With his mechanics, he'll sit consistently at 90-92 and have movement. Big question is durability. While he can touch 94+, he does't stay up there consistently. If his fastball were to drop down to the 88-89 mph range, his other pitches would become a lot more hitable. Still, I'd say right now, he projects as a number 3 with a ceiling of a number 2. And being 25, that's pretty great.
Sign him, give him the number 5 spot to build up his arm to MLB standards for a couple years. Move Detwiler to the pen, get rid of Mattheus, who becomes redundant as Detwiler is a groundball pitcher and a lefty.
Strasburg
Gonzalez
Zimmermann
Fister
Tanaka
Stammen
Olehdorf/Roark
Detwiler
Blevins
Storen
Clippard
Soriano
Leaves you with Roark, Taylor Jordan, Nathan Karns, & Dan Rosenbaum in Triple A as back up, with guys like AJ Cole, Matt Purke, & Giolito all able to work on their stuff in the minors with no rush to bring them up.
I realize that offense seems to be our biggest problem and signing Tanaka would probably eliminate signing Jordan Zimmermann long term, but Tanaka would still improve the team, dramatically and, most importantly, long term. Fister and and Zimmermann will both be free agents at the same time. Tanaka would slide right in to the number 3 spot with Giolito, AJ Cole, Purke, and a short term free agent duking it out for the final two spots for the 2016 season.
Offensively, we have to dump Laroche or pray he returns to career form. Still think it's pathetic that you know going in to the season that your first baseman shouldn't hit higher than 6th. Still think playing Zimmerman at first and putting Rendon at third and trading for a second baseman, or trading for a third baseman is the best move. There's no way Zim moves willingly until he finds out his shoulder is done. Werth isn't moving either. I'd really like to get Chase Headly. We know he can play third, but play him at first until we have to move him. His bat would carry, especially given his home/away splits. And if the Cubs are still interested in Drew Storen, by all means, get Anthony Rizzo.