Now we are getting into engineering of the pipe, and to me, using a plastic tube does not qualify the pipe as being "properly engineered."
There are very few things us pipe smokers will agree on, but one is a briar pipe requires proper curing and proper engineering to be a good smoker. I believe Savinelli offers the proper curing aspect, but falls short on the engineering due to the filter system.
You've got that almost completely backwards.
People put way too much emphasis on curing. The block cutters cure the Briar, that's all it needs, you can read about modern artisans using fresh Briar with rave reviews. Curing is one of the most hotly debated aspects of pipemaking.
When you're talking about the aerodynamic effects of airflow, I doubt there's any practical difference between having 20mm of space or 5mm of space in the shank, as most filterless pipes do. Either way you're dealing with a lot of turbulance from the transition from shank to stem and the entrance in the stem needs to be chamfered properly, that's pretty much universal no matter what happens before the air hits the stem.
On most pipes I've seen (around 20 Briars so far), that step isn't done (and Missouri Meerschaum plastic stems are nearly technically perfect). I usually take a large drill bit and spin it with my fingers to round out the airway entrance a little, it's the same process whether deep inside a 9mm filet slot or right on the end of a filterless tenon.
So many pipes have mismatched airways between the shank and stem (basically any pipe with any amount of bend) that I'd bet you're just as well off having a large buffer space as a small one. I'm temped to say that an empty filter slot is aeroynamically superior, but I haven't seen any pipes tested in a wind tunnel.
Savinelli gets constant recommendation because they have good QC and are readily available at affordable prices. I actually have three Brebbia pipes now and they're all fantastic, but they're not affordable unless you buy from overseas. Local dealers seem to charge a premium for most brands just because of distribution methods.
Many brands have good engineering, but Savinelli is set up for mass distribution in the U.S.