Pig fat, fouling spark-plugs and the air, washing cyl walls .... SHALL I continue ? ..
Then you need to get better at TUNING them!
I've had a number of 3310 Holley 750 vacuum secondary carbs on everything from a truck 360 to a mild performance 440 and they all peform really well with NONE of the above mentioned traits!
The 360 in my 4x4 powerwagon went from 9 mpg with the tiny 2 bbl. to 11-12 with the Holley. Not great but the thing weighs over 5,000 lbs. and is a full time 4x4. Starts instantly, no fouling, no problems.
The 440 in my '71 'Cuda got 13-14 mpg with a .528 MP cam, headers and Holley 750 on a regular basis. Not spectacular but it started instantly and had great all around performance.
The 318 with 360 heads that I built got 17-18 mpg with a .480 cam, single plane M1 intake, headers, etc. Again, not bad for the combo.
Doc can cry all day long about the Holley's but the simple fact is they are a great carb that is easy to tune and parts are as close as your local O'Reilly's or Autozone.
Thermoquads may have worked well back in the day but, as others here have pointed out, are quickly going the way of the dinosaur.
All those MPG figures you gave really suck compared to stuff I have had with thermo-quads so I don't get what you are bragging about
Half 2 agree with ya Dave, I do love my holleys no doubt, allways will, But 2 pair them up along with a TQ or QJ for gas mileage, The spreadbore will always win in that department propperly tuned of coarse.
75 Duster, 451 10.87 @ 123.58 NA
97 Z28 6sp., 12.01 @ 115 on a 100 shot
71 Swinger. 360 magnum. 12.58 @ 105
78 cutlass, 469 BBC. 12.70 @ 108 on street tires.
http://www.youtube.com/watch?v=Z2kqNmMfheU
....kind-of 'splains most Berri-Crabs .. 
