Depends on the motorsport and weather you have fences or not to deal with.
To shoot through a fence I find you need 300mm+ to help the camera not see it. I get more useable shots from my 400mm F4 prime then anything else I have used, I use it 1.4 and 2x tcs to great effect I think, and pretty much most from Brands Hatch have catch fencing in front.
I have tried canon 70-200 f2.8 but pretty much unused, 300mm F4, 400mm 5.6, sigma 150-600 but if I still had them all my canon 400mm F4 would be my go to trackside spectator lens, at Brands Hatch and Thruxton. Sharper and faster.
The 70-200 f2.8 would be used a bit more if I was trackside.
That’s on my full frame 1dxmk2
On my old body it would the 300mm F4 as the 400mm would be too long for druids.
So personally I think the answer lies in what your doing and what body your using as well as the track, as having been to Donington park and Silverstone you need as much focal length as you can get.
Primes do handle tc’s better too but I am firm believer you need to get them matched/calibrated to was how other and the body to get the most out of them.
My ideal kit bag as a spectator would be 24-70mm F4, 300mm f2.8, 400mm f2.8. Dependant on weather I was shooting cars or bikes, bikes are smaller.
Trackside at Brands hatch id take the 70-200mm f2.8 drop the 300mm f2.8 and take the 400mm f2.8, with tc’s. The sigma 150-600mm can’t touch that setup, in terms of autofocus speed and pixel peeping iq and therefore print sizes. But can cover most of the range.l, and as a whole weighs less because trackside you could do with 2 body’s so your not swapping to cover everything.
I took a couple years out for personal reasons where I didnt attend motorsport, when those prime lenses where all the rage with media, it’s why I first invested in my first prime.
But not many media photographers have that kind of kit nowadays. Most common bodies seen are prosumer body’s and zooms, and it’s noticeable in the events I have been to and seen the results recently. Some of the shots in some media iq is not what I would publish on my website.
I was once told once you go prime you won’t go back to a zooms. Pretty much right I did try the Sigma after it’s raging reviews, until a canon 400mm F4 prime come up, which blows the zoom away.
Modern zooms are good but still not as good as primes imho experience, but some media people still get media accreditation with zooms.
I’d assume zooms are the rage though due to lack of money involved in being media accredited, as speaking to some they would love to have the money to invest in the gear but are not earning much, which is common in sport photography too a point to. I was approached but had to turn down a media accreditation as it would have ended up costing me money that I can’t afford to pay out.
So I’d say primes beat zooms for everything tbh in pure speed and iq but it’s your budget that rules your kit bag. Who’s got £30k plus of (for the f2.8 primes,) spare cash compared to a canon 1dxmk2 and sigma 150-600 which can be had new for £6k. Because outside the biggys f1 Moto gp’s big agency’s you won’t make it back.