16G EvoIII will easily get you to 300 / 300 with proper supporting mods. 18G-XT will get you to the torque limit slightly later and will hold the max HP slightly longer.
I don't think the spool difference between a 16G EvoIII and an 18G is huge. You're talking about a pretty small difference, I'd go 18G-XT on a 2.5L.
If you're concerned about the tranny, use the tuning to keep you within your comfortable limits. Nobody is gonna force you to push the turbo to the max. Just make sure the tuner knows what your comfortable limits are. May cost an extra 1/2 hour to hour of tuning for some fine tuning of your boost curve that doesn't happen with all tunes.