Too flat, the top entry has to turn too hard to change to go sideways. The 2.3 runners are two short and two long like on 2.0 but the top is woefully incomplete to make the effective length even worse than the short ones on 2.0. Half of runner length is missing on top of runner, because plenum was crammed into it. Too much change in sharp angles, the 2.0 is all gradual curves like airflow likes. 2.0 also has a length from top into runner entry that allows air to curve without dropping out fuel, which is a major problem on these motors, the ports are too big in diameter, why they half-as-ed tried to improve it by going to D port. The D port extending into manifold takes what would have been a good idea and makes it worse. Or a nice flat floor for fuel to slam into and drop out of suspension with air. The bottom half of port literally flows no air at all, all flow comes at the roof. They were trying to shrink port to bring mixture speed up at low rpm by eliminating the low flow area. The 2.0 intake is so much better that it actually still improves flow even though the entry from manifold to head on the outside cylinders does not match, a big overhang there. 2.0 manifold actually outflows almost any aftermarket manifold made to fit the 2.0, even the hi-perf ones. Works very well on 2.3 too even though mismatched ports, but you have to use an adapter plate to fit it to the 2.3 head. 2.0 does not have the EGR plate that screws up airflow even further either. You can fit a Holley or Motorcraft 2 bbl. directly on the 2.3 manifold with EGR adapter removed but the resulting combo is so flat that it does not work either. No room for fuel to turn the right angle corner from vertical to sideways. As a result, the throttle bores with a partially open butterfly really feed some screwy flow patterns there. All engines like at least 2 inches or more to get past a partially open butterfly to straighten so it can make the turn into top of runners. Why spacer plates under carb can help like V-8 manifolds at that point. Adds distance to allow mix to make the turn easily without fuel drop out. Curve at top of runner entries helps too rather than a sharp right angle. The 2.3 manifold is an absolute disaster as far as that is concerned.
Hard to believe that engineers could so butcher a manifold on a simple four cylinder engine like that but Ford definitely managed to do it. It's like they did everything they could think of to make it not work right. They could have added another inch here and there and the manifold would have added ten HP to the engine by itself, or what the 2.0 intake pretty much does.
It's absolutely the design, no way on earth could you ever port that thing out to match the other manifold.