I can't comment on the flash, but the Canon 75-300 lenses are all very poor optically. There are alternatives that are much better, but we need some more information to give you the most appropriate solutions.
1. Camera or cameras are you currently using?
2. What is your budget?
3. What is your preferred focal range?
4. When you say racing photography, what kind of racing? Horse, yacht, motor, indoor or outside?
I have the 75/300 lense and a df-400 power extra flash. I shoot racing photography. I wonder what other lenses or flashes would help create sharper photos? Thank you!
The EF 75-300mm lenses seem to be designed to teach you about photography more so than capturing stunning photos. To be fair, though, you can capture pretty good still life images between 100mm and 200mm using a tripod using the center AF point to focus.
Using a flash is highly inadvisable for any type of sports photography. What camera body are you shooting with? What is the range of typical focal lengths that you use with the 75-300mm lens?
Help someone to help you. Post a sample photo of your work so that people can get a better “picture” of what type of results you are getting.
Does your 75-300 give you a big enough field of view at that distance? For example, the image you provided does not show the whole car.
If you look across the range of images you consider successful, what focal ranges are you shooting at?
Also, in part as a matter of curiosity, what is the reaction from drivers or the organizers to you using a flash so close to the racing?
"I have had no complaints of the flash usage as they are used to it!" Fair enough. Part of the reason I was asking was if you wanted to be able to be free of depending upon the flash, but I'll let that one lie.
Can you please advise what focal lengths you tend to shoot at, as opposed to the given 75-300 of the lens you are using. There is a point to this. If you shoot only up to say, 200mm, then we can look at a different class of lens to find you the best solution.
The EF-S 55-250mm may be another good option for them. But we can't jump to conclusions before OP tells us what focal lengths that their using. Before giving them lenses that are too long.
40D, 5D IV, EF 16-35mm F/2.8L III, EF 24-70mm F/2.8L II, EF 28-135mm F/3.5-5.6 IS USM, EF 50 F/1.8 STM
430EX III-RT, 600EX II-RT
I guess you missed my post where I said pretty much exactly that. However, it seems that the OP wants up to 300mm, so the 70-300 series will give them a little more at the wide angle end as well as far better optics and stabilization.