It would reduce the airflow, and I decided to go with the 18.8mm in the end. Thanks for pointing that out! I would have otherwise overlooked that important fact.
I've officially placed my order for this gorgeous recycler here:
http://instagram.com/p/nwbvXeS-Oa/
except I have asked him to put in a diffuser of his choice. I trust his personal preference when it comes to the optimal amount of diffusion for vapour, so he's going to surprise me on that one. It'll be 18.8mm, with the mouthpiece on a 45 degree angle like this:
The mouth-piece will also be made similarly to this one here: (photo is actually of a Sheldon Black piece)
Also for the middle of the base, the bottom window and the base wraparound he is incorporating colours similar to these to really bring it all together:
And Its not going to cost me more than like $200 USD (and that's with postage to Australia included)
If any glass enthusiast reading this has not already contacted this incredibly talented, friendly, persistently helpful and highly skilled glassmaster then they are going to regret it when this guy gets the attention he deserves and blows the fuck up and starts charging $600+ for custom pieces!
I scoured AquaLabs, Grasscity, OzBongs, DreamHerbs and many others for 2 days straight and THIS is just.. the best.
I've said enough.
Im obviously excited.
I've also encouraged him to start using his account to visit these forums and he said that he might start coming here to answer questions / deal with his FC customers