What expectation? Apple tech specs are very brief for the ssd drives, just size and 'PCIe based', and they have a nebulous comparison to a spinning drive. They also don't tell you what model processor you get, just family and speed.
The expectation is that a 2014 model will perform at least as well, if not better than, a predecessor model from 2013.
That's a perfectly reasonable expectation to hold, as well. One really shouldn't end up worse off (for whatever reason) merely by buying a more recent model, especially when it's a computer that's being purchased.
