A working and calibrated vacuum advance will make a huge difference as the rpm rises.
At cruising rpm, say 2600, your engine could be wanting high 40s to mid 50s for advance. By 2800 it might be wanting hi 50s.
If you don't give her what she wants, the fuel charge will begin burning too late in the cycle, and will not finish burning before the exhaust valve opens. Therefore, it will have to finish in the headers.
This causes several problems;
Firstly, the headers run very hot.
Second, this destroys the scavenging effect of the headers.
Third; if the header flange is not tight, and oxygen finds it's way into the header tube, you will experience after-fires, but
Fourth and the biggest issue is that optimum torque cannot be achieved, so you get terrible fuel mileage.
To illustrate this;
Start your engine up and get her warmed up. Put her up on the fast idle cam around 2400 rpm, exact rpm not important so long as it is about 400ish rpm below your cruise rpm. Read your timing.
Now, just tug on the Vcan and start advancing it. Keep on advancing until the rpm no longer increases.
Now read your timing again.
Then return the timing to where you found it, and kick it off the fast idle.
So what did you find?
The increasing idle speed showed you that peak torque was not being generated until you gave the engine the timing it wanted.
Understanding that, it is no stretch to imagine that with the correct amount of cruise timing, your throttle opening can be reduced... which should reduce your fuel-delivery, so now making the same power on less fuel, so up goes your fuel economy, and down goes your operating temperature. BadaBoom!
So now, it remains for you to establish the correct cruise advance for your engine, and to engineer it to happen.
Furthermore, you can do the same thing at all popular cruise points; like 30/35 mph in second gear, or 45/50 in third or just pick any rpm of interest to you, and calibrate your curve to deliver, as close as you can, the advance she wants.