I am getting 88.75 for sensitivity for this driver using WinIDS Pro as well. Perhaps Winisd is changing the values after inputting everything in? At any rate, here are some quick models for you to compare to what you are getting. Grey is a 4.5 cu ft ported sub. Pink is a 6 cu ft ported sub. Yellow is a sealed 4 cu ft sub. All subs take into account 200 watts, and the ported subs take into account a 20hz tune and 6" ports (you can model 4" ports if you would like). Also all stated sizes is internal volume only. Remember, you have to factor in material thickness, double baffle, bracing, and woofer volume.
As you can see the 6 cu ft sub has more SPL at the tune.
Here is how the above subs look when you build four each (4 x using the same parameters above for each sub):
I think the limiting factor with the sealed subs is the amp. Here is what happens if you put 500 watts in each driver in a 4 cu ft sealed box (yellow) vs the other ported subs using 200 watts per sub:
Just my opinion for what it is worth. I think if you are limited by box size (a 6 cu ft internal box is not small) and have to build smaller boxes under 5 cu ft, you should consider selling the 200 watt plate amps and getting a beefier pro amp like an iNuke 3000DSP to power all four. They cost around $300-350 new (I got mine for $290 shipped brand new in December). There are other amps out there to consider as well. Again, just a thought, clearly you can make ported subs work with those amps, but you have to consider box size.
Hope this helps.