Performance is specified over a range of temperatures and the tools are designed to place components in paths that will meet these specifications over that temperature range. Once you have a design, you cannot make changes and compare the new results to old ones -- the critical path may be different in each case. You would also need to operate the IC out of spec, >80C.
If you want to get the IC hot, simply make a design with all IO as SSTL DCI inputs (or SSTL2_II outputs...) The DCI features on older Xilinx parts implements SSTL outputs in a power-inefficient mode. (GTL/HSTL likely work as well). You might have to mix these based on the bank voltages.
Beyond that, you can just create some complicated, very pipelined, hashing function and use it to hash MB of data at a time and then output the results over some other interface. The hash can also be computed on a computer, and used to verify the results.