...As far as HOLLEY Carburetors, I have used both Mech. Secs. and Vac. Secs, for Decades.
On the Pantera, I did a custom machined 'Dual Quad' Mid- High Manifold (IR, 2x450's), I opted for Vac. Secs., should You want to Talk about Throttle Response. Yes! Mechs come All-In, All 4 Bores, 2 Accelerator Pumps, when You 'Punch It'. Response doesn't get any Faster than this! A Liken it to "...A Sledge-Hammer in Your Backside". As well as That, TOO Large a Carb can Also, Cause a Bog, at Lower RPM's. That's Where Vacs work best, They Come-On Only When Needed and Give Only the Amount of Fuel/Air that Is Needed.
I did Dual Quads on a 400 Pontiac (Dual Plane Weiand) using Mech. Secs. (2x390's) I do not 'Track' any of My cars. My work was always for the 'Street'. I Commuted the 400 Pontiac to Work, Every Day, for 30 Years!* I now run exclusive Vacuum Secondaries, on a Edelbrock 'Air-Gap' Manifold. When I drove through the Desert I was using a 600 CFM. I have since gone Back to the 750 CFM, both have Vac. Secs. I Missed the 'Greater Roar' of the Larger Venturis.
My Point...Yes, greater response with the Mechanicals, but My opinion is, they should Only be used in Racing on the Track. They are meant for all out competition, they are Always 'ON' as for Fuel Flow, especially 'At the Idle'. Copy, Fumes of Unburned Fuel. Plus, You will have a Greater 'Chance' of ever Passing a Smog Inspection, when You are using a Vacuum Secondary Carburetor.
I believe You will be Most Happy with Vacuum Secondaries on the Street.
Although, the Response from the 600cfm, 'Seemed' more 'Crisp', than the 750...the Exhaust Tone of the 750, was a Greater ROAR, when the Secondaries come On. Both used on the same air-gap Manifold. Single Carb, Dual Plane.
Last...if one was concerned with the 'Miles per Gallon', they would Never have purchased a Pantera!! The fact is Mech secondaries are 'paid for' with Excess, Wasted Fuel...All The Time! Even while 'sitting at a 'Red Light'. They belong on the Racetrack Only.
Just My Opinion.
This may help You with Your decision, Good Luck with It!
*...gonna sound like bragging, I NEVER passed on a 'Red-Light' Street-Race, or Any Race! A Highly Modified Pontiac, Dual-Quad 400c.i. in a 6.6 '78 Trans-Am. Near 500 HP, Weight-Stripped Off more the 800 Pounds, ALL the Suspension Tricks, in the Book, and then some...Was Undefeated in all those Years of 'Street Racing'! That Machine had a T-10 4-Speed, You could 'Slam' the Gears, In. Can't do this on a ZF!! With Mech Secs, I could Easily bring the Front-End, Off the Ground, and Only Move the Car Forward, One Inch! Traction-Bars, 'Torque-Chain'!! Don't know what a 'Chain' is? You've never competed in Drag-Racing, or You used a Torque Plate. Why did I Not take it to the Track?? TOO MANY RULES! That TA was so Highly Modified, it was deemed 'Illegal' on the Racetrack, and was Banned!
'DO NOT TRY THIS AT HOME'!! In California, it can get Your Pantera, CRUSHED! The Gavel will Fall!
MJ
I was sitting in My Lane at a Red-Light Stop. The Other car was sitting at my right, in the 'Right Turn Only lane'. The Driver planned to Go Straight, jump-out, and cut Left into My Lane, Cutting Me Off! Imagine That...an Illegal Act. LOL
When the Light Turned Green. We Both 'Dumped the Clutch' for All She Was Worth. He wasn't even Close. Remember, more that 800 Pounds stripped off that Machine and the Horsepower, Doubled.
I can remember, glancing in the Rearview Mirror, after Leaving Him Behind...The Owner of the $120,000+ Porche was So Pissed-Off, Pounding His Fist into the Dash! When he soon came to his senses...He realized his Committed, Full-Bore, straight-shot launch, was heading toward the far corner/sidewalk and a Telephone Pole. He locked up the Brakes! Second Place is a Bitch! Better Go Get Yourself a Pantera. I built This Car, I built This Engine. Yes! It was the 400 with Twin 390's Sideways on a Offenhauser Low rise. Dual Quads with Mech Secondaries. As I said...Racetrack ONLY!