On my 2013, and I think the '14 is the same, the grill seems to be part of the speaker. Only the surround can be easily removed. So based on that I'd say no, you can use the '14 door sub grill on an '08. Maybe you can fit the whole assembly in though? There are probably a few "take-offs" available.
2013 GT Premium Brembo
MGW, Blow Fish, AM Hood Struts
Tinted, Borla Touring AB, ProCal Tune
Full Shelby/Kicker Audio Upgrade