I would be surprised, too. That would mean each iPhone model has the same RAM from the 6 plus down to the 5C?
However, Apple did already differentiate on other aspects between the two. I am naive on this subject, but how will the two models have equal performance with the full HD resolution on the Plus.
Also, landscape mode on Plus seems to lay groundwork for split screen apps if that gets released.
RAM won't make a difference in performance. RAM helps on a regular computer because a regular computer will "swap" some of what is in memory to the disk when the memory gets full. Getting this data from disk back into memory can be excruciatingly slow. iOS doesn't do this ... it just terminates background apps as memory fills up.
As far as performance goes, it is possible that the 6+ might have a higher clock frequency which will make its processor (or the GPU) run faster. This will cause the battery to drain faster, but the 6+ probably has a much larger battery so the extra drain will not be noticeable.