I previously owned an RX7 cooler. Maybe it varies by year, but the one I had was quite large. Theres no way I could have gotten it behind the grille without some work. Then it would have covered up radiator quite a bit.
After talking it over with Mike, it looks like the stock cooler is quite a POS. Anything of decent quality should be better so I may just pick up a 12in x 12in or something from B&M. I dont know yet... I think as long as its made by a reputable company it should exceed the way the OEM piece works.
I wanted to get a trans fluid change so I may go into the local trans shop and see what they have for coolers. Then I can get a 1st hand account for size and what will fit behind the grille while being big enough to cool nicely.