mid-2015 MBP, 16GB, upgraded from 2.5 to 2.8 model. Benchmarks are almost the same, even worse for the 2.8 in some tests
Have Intel Power Gadget running while benchmarking, and watch the temperature and frequency charts on both macs. If your 2.8Ghz Mac has cooling issues (poor airflow due to dust build-up, bad/failed thermal interface between chips and cooling system) then you'll probably the clockspeed (frequency) drop under load to keep temps under control.