I have a dart small chevy 377 shp build and run aroud 9.8 to 1 compression ratio (4.155 bore x 3.48 stroke) with flat top pistons 2 valve and dart pro 1 200 cc platinum aluminum heads all ported out and gasket matched and worked over very well and run a edelbrock rpm performer air gap intake and a 220/
[email protected] 510/510 lift with a 114 lsa and I won't bother with the rest and its in a 96 chevy s10 with a 3000 plus stall convetor and turbo 350 and I can run a 650 double pumper or a 650 vacuum secondary and I get 19 mpg on my build going down the highway at a steady speed of 60 mph at 2800 rpm if I keep it that way and slightly less at 70 mph at 3200 rpm. I also ran a holley 750 HP which is for all out performance and still got the same mileage with proper time and tuning.
Holley carburetors will never do as good as a Qjet for mileage since they have smaller primaries for better fuel atomization and throttle response but for all out performance its my experience and just my two cents that a holley carburetor will outdo a Qjet for overall all around performance any day of the week and that is why most people use them and a Qjet was never meant for all out racing motors which is the reason why they don't get used as much as they were meant more for everyday driver cars and trucks etc.
I would check out holley store on ebay and look under there refurbished carburetors and they have some good deals on the quick fuel slayer 600 vacuum secondary carbs at times for around $250 bucks or so and some of the holley brand 600 vacuum secondary ones can be a little cheaper yet. They also have some 750 carbs for around $300 and are great buys. They are mostly ones people bought and did not have them adjusted right and thought something was wrong with them and they just did not know what they were doing and holley will resell them for a cheaper price since they were installed.
A holley can be made to do decent on fuel mileage if you learn how to tune one and know your way around them. My brother once had a bone stock 305 small block chevy in a 88 chevy caprice and his carb was shot and I replaced it with a holley 600 vacuum secondary which is not the best for mileage compared to a Qjet but with some time and tuning I got it pretty close to almost making the same mileage as his Qjet did when it was working good before it went downhill after getting worn out.
His Qjet had some things so worn out on it at its final years that it would have cost more to fix it then what it was worth so I bought a used holley from ebay and fixed it up for the cheap and it was in good shape still.
Just my experience and from reading on other racers out there that have went both ways with carbs and different brands and styles. I am not saying I am right as its just my opinion as all carburetor designs have there pros and cons when it comes to tuning and there design and ease of making them perform more then what they was made for.