I have looked into this topic pretty extensively...
First off, a throttle body should not be compared to a carb
Ever heard of the Bernoulli effect?
Dynamic (moving) fluid = Lower pressure
Static (no movement) fluid = Higher pressure
In other words, faster moving air creates less pressure than air with no movement. Take an airplane wing with the flat bottom, and the top part of the wing is curved.
In order for the air to get from the front of the wing to the back of the wing at the same time as the air on the bottom flat part of the wing, the top air needs to move faster. This creates low pressure on the top part of the wing, which creates a higher differential pressure on the bottom. This causes lift, and makes airplanes fly
Ever been in a house with the windows open? As the wind starts blowing outside, your drapery will start "sucking" towards the outside. This is because the air is moving faster on the outside, than inside. It creates a higher pressure on the inside of the house and pushes outwards until the wind dies down and creates "even pressure."
So what is this getting at?
Air gets into the combustion chamber due to atmospheric pressure (14.7 psi). The more pressure (less air speed in the throttle body runner/plenum), the possibility of more power.
A large throttle body is not going to hurt your performance.
It will give you one thing, that can be described in two different ways:
- More throttle response
- Touchy throttle response
They are the same thing, just worded differently.
Remember, that not all throttle bodies are created equal. One 70mm throttle body, may not flow like another brand of throttle body of the same size.
The design, shaft thickness, and screw protrusion are all pretty big factors in this even though the blade itself may be 70mm.
As far as area, just multiply
PIE x Radius (squared).
60mm - 4.4" ^ 2
70mm - 5.9" ^ 2
80mm - 7.8" ^ 2
90mm - 9.8" ^ 2
For every degree the throttle is pressed under wide open throttle, the larger blade has more surface area to pass air. This is what gives the extra "throttle response."
I believe the term "overkill" is correct, if you use it correctly. It is not overkill in the sense that it will hurt power, but it may be not needed.
Anyways, not lets look at the CFM the cubic inches can get you at a certain RPM and efficieny level.
Displacement * RPM * VE / (1728*2) = Volumetric flow of the engine in CFM
The 1728 converts from in^3 to ft^3, and the 2 comes from dividing RPM by 2 in order to get the actual number of intake strokes. Many dynos will configure the VE for you.
So take 281 * 6000 * 1.0 / (1728*2) = 488 CFM
Now take a look at the throttle body flow rates from Accufab:
708 CFM at 28" of vaccum.
Now an engine does not see 28" of vacuum 99% of the time, but you can get an idea that the flow is typically not an issue.
Most "gains" are seen by the different variables that can occur.
Anyways, a large throttle body is not going to hurt your straightline performance one bit
How can having to much air hurt you at WOT?
If you want to throw a 75mm on, do it. If you want to throw on a 70mm, do it.
If you've got a throttle body that delivers 100% of the peak air requirements of your engine when the throttle plate is fully open, you have control of the air throughout 100% of the throttle position range. If you go to an oversized TB that delivers 100% of the air that your engine can consume while the throttle plate is only 60% open, you have given up usable throttle-control range for no advantage.
It is your money
Just keep in mind, a larger throttle body is not going to hurt you at the track.
Good Luck.