Did you ever resolve the FFT v7.1 vho problem?
I have been having problems with the FFT v2.2.1 vho so I tried updating the FFT Megacore to v7.1. I get the problem you are having with the maximum exponent (-16 in my case) and the SOP/EOPs are not correctly generated.
Post synth netlist doesn't work properly when I create a netlist for just the FFT itself, and if I create a post synth netlist for full signal processing block ModelSim runs out of memory (I have 16 FFTs). The only way I am currently able to simulate is to generate I/O in ModelSim, export WLF to VCD, perform a Quartus simulation, export the results to VCD, then convert to WLF. It's not a quick or pleasant way to simulate.