I just checked on the Datrats website and you are correct - the FIA L-series head appears to have round exhaust ports. But Nismo A-series manifold gaskets still have sqaure/rectangular ports.

I can't really imagine how a square exhaust port is cheaper to manufacture than a round one. I've certainly seen US heads with rectangular intake ports and I think round exhaust ports ?!?

Some non-crossflow heads are designed with pairs of intakes and pairs of exhausts together E-I-I-E-E-I-I-E, which is in contrast to E-I-E-I-E-I-E-I. The main advantage I see is that each intake port is heated by only one adjacent exhaust port not two. This helps keep the intake charge temp down. So I could understand the engineers making the ports rectangular if it means you can jam them closer together and leave more space between exhaust and inlet ports. But it would not appear to be the motive in the design of the A-series head.