When implementing a half-band filter (because of the coefficients I am loading, I have selected 'Half Band' symmetry), the filter is using too many DSP slices.
Sysgen is reverting back to a 'symmetric' implementation of the filter instead of half-band, which I had applied in the properties of the IP core.
Instead of changing to Symmetry implementation with no notification, the IP cores GUI should return a DRC error when I try to select a half band filter implementation with illegal coefficients.
This is a known issue which has been fixed in Vivado 2015.3.
A DRC is returned when entering the values in the IP GUI.