Is FM assuming we're using the oem cat and exhaust? I don't understand the purpose of the hybrid DP. Can't you just use a high flow cat that's 2.5" instead of 2"?

From FM on their Hybrid DP:
"This special downpipe allows a 1.6 car to be fitted with a 1994-97 catalytic converter. Why? Because the 1.6 cat has a 2" flange on it that forms a big restriction. This hybrid pipe maintains the flange size and location of the 1994-97 cat so you can use any aftermarket or stock cat and remove that bottleneck. Exclusive to Flyin' Miata!"

 

The inlet flange on the 1.6 cat is significantly smaller than that used on the 1.8, and you can't fit a 2.5" pipe in it. So if you want a cat that's truly 2.5", and not 2.5" with a 2.0" inlet, then you have to use the 1.8 flange.

That's why the downpipe exists.