I'm sure its about what the cylinder heads flow stock and what the current turbo's can supply. 1800cfm (if thats correct) is a lot of cfm! If all bottlenecks are taken care of before and after the heads and the stock turbo's reached a point were you didnt make more power per adding lbs of boost then you could look into ported heads. Once you add ported heads you'll loose boost b/c the heads are more efficiant (flowing more air) so then you'll have to add bigger turbo's to get the needed volume of air back.
This is what I know from dealing with forced induction applications on cars. You can only force so much air into the cylinder. Ported or not. When it comes to cost, for most applications minus all out race trucks or guys looking for bragging rights on the dyno. Ported heads dont yeild the power/dollar ratio most like to see.
You have to get rid of all the bottlenecks before/after the heads. Once the heads become the restriction then look into the porting. I know diesels are kinda different but I'm sure the same "recipe" applies. I'd assume more fuel, bigger turbo's would be more beneficial for the average guy. Cost wise.
Setup on my vette made 800 rwhp at 14lbs of boost. (supercharged application) Going to a smaller pulley made more sense to keep making power at my power level then adding a set of ported heads. Its a balacing act. You'll eventually have to add better flowing heads to keep gaining power. But then I'd need a bigger blower. Pulley=$100 bucks compared to porting my existing LS3's or new TFS heads was $1000-3500. Ported heads got me less boost, maybe a little more power compared to adding a smaller pulley and gaining more power. Power/dollar comes into play. I know you cant pulley down a turbo but you get what I'm saying.
All in all I couldnt imagine a set of correctly ported heads to be on the list for the adverage DD wanting to make good power. More of a dyno queen, race application.
Just my $.02
There's really a lot of variables is what I'm trying to say. I dont see it being worth adding ported heads to a stock turbo application. But with aftermarket turbo's...sure.
Now I'm pretty new to diesels so if someone more knowledgeable with diesels see's a discrepancy with anything I said feel free to correct me.