The SPI-4.2 v8.1 Core does not support back-anno timing simulation on the SPI-4.2 design targeting Virtex-5. The timing simulation file generated gives incorrect simulation results.
To work around this problem, re-run NGDBuild through back-anno with the following environment variable:
XIL_MAP_NO_RAMB_UNTIE set to "0".
This issue is fixed in ISE 8.2i Service Pack 2.