The engine I have in my daily driver KB310 has a D ported inlet A87 head with 44mm inlet and 37mm exhausts both in 214N stainless. This head flows 206 hp, the FIA flows 222 hp.
When used with carbs, the FIA ports are too large for low low speed operation. Slow gas velocity allows insufficiently atomised fuel to drop out. This with the lack of chamber swirl causes stumbling and bad throttle response at low rpm. This is where the bad road reputation comes from. With injection these problems disappear due to better atomisation.
Port angles on inlets allow for much better cylinder filling at high rpm. The exhaust ports are very good as well.
These heads in showroom stock condition flow as well as the best modifed A87 heads. When an FIA is modified it is awesome.
But I am biased