To follow up:
Had the radiator installed the other day, and it's been running perfectly for the last 2 days.
Mechanic said that since it seems to provide the same performance as the 27mm core, that possibly Toyota 'over engineered' the original core, and that this 16mm one should perform fine under normal driving conditions. Or that maybe the 16mm core is a 'revision' developed to save on manufaturing costs. All makes sense to me, either way, problem solved!