About to be Wladimir Balentien.
I encourage everyone to look at the aging curves for Mike Cameron, Shane Victorino, Randy Winn, Johnny Damon, Kenny Lofton, and Torii Hunter. Specifically, what their ISO\wRC+\wOBA\UZR\RngR were from age 28-32, and then what they were from age 33-37.
cain is only 31, so next year is his age 32 season. I'll go with the 4yrs up to age 31, and he 4yrs after.
(since the 5th yr is just a sweetener anyways).
similars from the last 25yrs or so
Age 28-31
B.Jordan 1863pa, 118wrc+, 17.5war, 6.1war/650
L.Cain 2185pa, 114wrc+, 17.9war, 5.3war/650
M.Cameron 2447pa, 104wrc+, 20.0war, 5.3war/650
D.White 2514pa, 104wrc+, 19.8war, 5.1war/650
K.Lofton 2527pa, 111wrc+, 19.2war, 4.9war/650
I.Suzuki 2954pa, 117wrc+, 21.0war, 4.6war/650
R.Winn 2720pa, 115wrc+, 17.1war, 4.1war/650
C.Granderson 2613pa, 119wrc+, 16.3war, 4.1war/650
A.Soriano 2685pa, 115wrc+, 16.5war, 4.0war/650
S.Victorino 2594pa, 110wrc+, 15.3war, 3.8war/650
J.Ellsbury 2095pa, 100wrc+, 11.7war, 3.6war/650
J.Damon 2782pa, 110wrc+, 13.9war, 3.3war/650
T.Hunter 2246pa, 111wrc+, 10.4war, 3.0war/650
well, "similars" is generous - he's been better than almost all of them and much better than a bunch of them.
Age 32-35
I.Suzuki 2915pa, 113wrc+, 21.4war, 4.8war/650
J.Damon 2525pa, 116wrc+, 13.3war, 3.4war/650
M.Cameron 2136pa, 113wrc+, 12.5war, 3.8war/650
K.Lofton 2388pa, 103wrc+, 12.0war, 3.3war/650
B.Jordon 2302pa, 105wrc+, 11.8war, 3.3war/650
T.Hunter 2409pa, 120wrc+, 11.2war, 3.0war/650
R.Winn 2552pa, 94wrc+, 11.0war, 2.8war/650
C.Granderson 2214pa, 116wrc+, 10.5war, 3.1war/650
D.White 2011pa, 100wrc+, 9.3war, 3.0war/650
A.Soriano 2081pa, 104wrc+, 7.6war, 2.4war/650
S.Victorino 869pa, 102wrc+, 6.0war, 4.5war/650
J.Ellsbury 1035pa, 95wrc+, 3.6war, 2.3war/650
Seems like most all of them aged very well, actually. A couple guys did get destroyed by injury, but otherwise most of them performed well enough to justify the kind of deal Cain is rumoured to be getting.