[AlphAcorn (OP)]

Having studied the extinction event of 65 million years ago, surely if it was a comet and not an asteroid, surely it would have an effect on the atmosphere as it approached being somewhat reflective, depending on it's trajectory on the surface of the Earth, like scorching the plant life etc, would this also have been a contributing factor to the extinction event?

[Janus]

Having studied the extinction event of 65 million years ago, surely if it was a comet and not an asteroid, surely it would have an effect on the atmosphere as it approached being somewhat reflective, depending on it's trajectory on the surface of the Earth, like scorching the plant life etc, would this also have been a contributing factor to the extinction event?

 The absolute magnitude for a comet might be ~3.5, compared to the absolute magnitude of the Full moon of 0.25 (In this case absolute magnitude is computed as what the apparent magnitude would be at a distance of 1 AU.0
absolute magnitude is a logarithmic scale, with luminosity going up as the magnitude number decreases.  By this scale, a comet passing as close to the Earth as the Moon would be ~1/20 as luminous as a full Moon, and thus would have to pass at between 1/4 and 1/5 the distance of the Moon just to be as bright as a full Moon.  By the time it got to with 1 earth radius away from the Earth, it would only be adding about as much light as you get in a typical living room*.  Not exactly planet scorching.
 And because the likely relative speeds between the comet and Earth, it isn't going to speed that much time close to the Earth. (At a mean relative velocity of, say, 15 km/sec, it would only take the asteroid ~ 7 min to cross that distance and hit the Earth. )

* Though by this time, the Earth would be well within the coma of the comet and the light reflected onto it from the coma would be much more diffuse and might not even add up to this much. 

[evan_au]

A comet glows by reflected sunlight. But the tail of the comet is very thin gas and dust, so it reflects only a small fraction of the sunlight falling on it, so it is much dimmer than sunlight (comparable to moonlight, as Janus calculates above)..

The Sun doesn't scorch the ground, and the Moon definitely doesn't scorch the ground.

On the other hand, a rocky or icy body 10km across would leave a scorching trail through the atmosphere, and throw lots of dust into the atmosphere - perhaps even putting some of it into suborbital trajectories that would come raining down around the Earth. This definitely would be a scorcher.

[CliffordK]

I would think that an actual impact would be relatively quick...

WHAM

Although, I suppose the comet tail may end up sweeping across the earth for a while so there might be some non localized effects.  But, once the particle density drops, very little would impact the surface of the Earth.  I.E.  spectacular meteor showers burning up in the sky, but little impact on the ground.

The direction of impact may not be directly into the Earth, but more of a tangential impact, or perhaps a curve with the air slowing and fracturing the comet until the main parts fall to earth.  So, perhaps some non-localized effects.  But, the atmosphere is relatively thin, so there would be very few trajectories where it would deeply penetrate the atmosphere without skipping off or directly impacting.

[evan_au]

In 1910, Earth passed through the tail of Halley's Comet, with no ill effects.
Halley's Comet is quite large, at about 11km across. This is similar to the size of the object which crashed into the Earth.
See: https://www.wired.com/2009/05/dayintech-0519/

We know that an object actually struck the Earth; there has been recent exploratory drilling into the Chicxulub crater.
See: https://en.wikipedia.org/wiki/Chicxulub_crater

[CliffordK]

Aren't the annual meteor showers related to comet debris?

https://en.wikipedia.org/wiki/Perseids
https://en.wikipedia.org/wiki/Leonids

Without extinction events. 

Yet, the density of the debris showers following a comet impact would be higher, as the nucleus would directly impact, the coma would hit a broader area, and the densest part of the tail would sweep across the planet.

Nonetheless, the major damage would be the impact of the comet nucleus, and the dust & debris stirred up by it.