Math blocks that provide output type conversion must pipeline the converter. For example, with AddSub, there is the ability to select 'User defined' output via the Output type tab. If you select this option, and even though the latency is set to 3, this does not pipeline the converter block.
To work around this issue, use Addsub with full precision and an external converter block with pipeline.
For the System Generator for DSP Release Notes from other versions, see (Xilinx Answer 29595).