The Sln_xferAck output is not registered in version V3_01_A of the OPB IPIF. This causes long timing paths when it is used with MicroBlaze, which makes it difficult to meet a 100 MHz requirement.
This problem has been fixed in the latest EDK 8.1i Service Pack, available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jsp
The first service pack containing the fix is EDK 8.1i Service Pack 1.