Ok, hypothetically, I don't know!
Let me think out loud, here, and you'll understand my point, I think.
The reason you're using the 16 now, is that it provided quicker spool... It does that by providing a little more restriction on exhaust flow, making more velocity past the turbo wheel, causing it to spin faster, sooner.
Since you went from an 18. 5 to 16, you made a moderate difference. I see your boost numbers are not real high (especially for all the mods, it looks to me like you should have more HP than me, for instance, especially since you appear to be able to run about 150-200 rpm more), they seem a tad low.
Now, EGT is factored by 3 things... . Fuel, air, efficiency (timing). Now, airflow is a function of two things - the ability to get the air INTO the engine, and the ability to remove the hot exhaust... You can't blow into a coke bottle - there has to be somewhere for it to go - so what you REALLY need to know, is this: If you change the housing, which will spin the turbo harder, will the restriction in your exhaust flow offset the increased boost, and you actually flow LESS air through the engine, but have more PSI boost, or is it sufficient, and the added boost will result in more airflow and cooler EGT, in spite of the increased restriction?
Honestly, I can't answer that question, because I simply have no idea. I have NEVER seen an engine with the NW 14 housing, much less have any knowledge from testing or even reported experience. I've never seen the NW 12 housing, either.
Now, I'm going to go off on a leap of faith and guess you're getting that housing from Piers (he's the only one I know that's actively marketed them at all), and Piers probably has the broadest range of experience and can make the best educated answer.
But, if it were ME... I would choose the Wastgated 14 housing, and make the best of both worlds. Once you achieve the boost you desire, opening the wastegate stops the increase (in fact, may decrease) the restriction on exhaust flow. How much does a wastegated 14 flow? With the 'gate open, I'd guess it's MORE than 16. Even if you gained not a single PSI of boost, you'd flow more air, reduce smoke, and lower your temps a wee bit, not to mention, gain that low end "snap" from the smaller housing and quicker spool of the 14.
Now there, that was a long, hairy answer when I could have said "I dunno, ask Piers". But it would have seemed rude, at least in my book
I see I failed to address the milage issue. I would guess that there are so many factors larger than just that exhaust housing, that you're not really going to notice an economy change, one way or the other - if you get more HP and use it, well... YMMV.
I went directly from the 18. 5 stock housing to the WG12, and an open exhaust and then I added PW injectors.
My best milage before those changes was almost 22 on the highway in the 60-70 mph range. My best since has been about 20. But my "normal" fuel economy has changed only a little, perhaps 1 mpg less. This could be because I'm churning a lot more heat in the converter than I used to, or it could be the engine is less efficient under cruise conditions.
On the other hand, the WORST I've done is before mods, as well. My first tank of fuel leaving Nevada got only 15 mpg. I've not done that bad since. I expect about that towing the trailer on vacation... but we'll see and I'll say when I get back.
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