According to DiRT 5 lead designer Mike Moreton, that’s not going to be the case. Speaking in any interview with GamingBolt, when asked about the difference between the GPUs of the two next-gen consoles, Moreton said, “From development and gameplay perspectives, it doesn’t really have an impact. When it comes to tuning, tweaking and optimizations at the end of a game, it might mean slightly less work for one platform.”
So basically development effort for both platforms should be identical. He did say that "When it comes to tuning, tweaking and optimizations at the end of a game, it might mean slightly less work for one platform." and I'm sure he's talking about the PS5 here. So the phase where they polish the game might take slightly longer on the PS5 since the Xbox has the extra performance.
It'd be interesting to see if there are other devs who share this sentiment or if it's just their experience with their game.