I had a similar low IOPS issue, but only on write, the read IOPS was fine.
It turned out that when I cloned my 2tb HDD to the 1tb EVO 840, it didn't align the partitions properly and caused a performance issue.
My HDD partition was aligned as I did it manually ages ago. Due to the partition not being aligned on the SSD it caused my read performance to be lower than it should have been, but the write speed was fine. Read IOPS were fine, but write IOPS were very low.
Once I aligned the partition everything was great.
Here is my before and after aligning.
Here is a nice easy guide to follow if your partition need aligning.
http://www.overclock.net/t/1226963/how-to-properly-re-align-your-ssd-hdd-partitions
I would highly recommenced using the Mini Tool Partition Wizard to align the partition as I find it's the easiest to use.
Make sure to backup first incase anything goes wrong.