It does seem like your transfer rates do not hit the lower end of the performance gains but then again, I would question your methodology. I do not see how a drive that has 1TB would be of any help and more so would be a hindrance to getting accurate results. Inevitably, there would be fragmentation that would of occurred on the old drive making it a less than optimal comparison to a new drive.
As for IOPS, you don't mention your computer's specs but the testing here was done on a pretty fast and efficient setup in that linked article and that also might contribute to the discrepancies. Also, we're talking about a benchmark setup with minimal background services running and meant soley for this purpose.
Ultimately, I would agree with your thought of not needing an upgrade if you have the previous generation but this drive does seem pretty awesome for a first-time purchaser of a large capacity drive.
As for other benchark software, I think Atto is free along with CrystalDiskMark. I don't know how they compare to other benchmark software and another thing that might make a difference is the type of test data and setting which I know can be changed in CrystalDiskMark.