My experience is different with th BMC CDA on the 1.9 JTD 8v engine. I had a similair airbox position as Dusan, but I had no gains in terms of airflow and boost compared to my modified stock airbox with 16v snorkel. Did a lot of logs at the time.
I did experience a small difference in driving, with the standard airbox it felt stronger in the 1000-2000 rpm (prespool) area.
Now with my 240+ hp 1.9 JTDm 16v, I also modified the standard airbox and intake manifold and the difference is more or less zero.
I'm confident the stock 1.9 JTD intake system is a very effective design with no restriction even with 100 hp over the stock output. Looking at the picture of the 2.4 20v design, to me it looks even better than the 1.9 system with a more straight flow through setup with few bends.
Sure, if you're after a sporty sound or cool looks, get the BMC, but if you're interested in performance only, leave it as it is.
147 JTDm 16v - GTB2060v - 329 hp/578 Nm