Buying better equipment is always advisable if you can afford it.
Only if you have no other use for the money, the impact on your images is sufficiently large, and there are no downsides to the purchase (like doubling the weight of the lens)
Couldn't you try this? Shoot with the f/4 but with ISO bumped up by a factor of two and see how often you have to go wide open. This would not tell you whether you would benefit from 2 of the benefits of the 2.8--narrower DOF wide open and faster AF performance with some canon cameras--but it would tell you how much the extra stop is worth to you. Would that be enough to settle it for you? (Full disclosure: I have the lens you have and would not 'upgrade' because for my purposes, the extra weight is real minus, I have yet to need the speed, and the money could be spent on other things that had a lot more impact on my photography. However, I don't shoot sports, so this may have no bearing on whether the trade would be a good choice for you.)