Does 500g of backpack weight really make such a huge difference?

I'm thinking about the Camino de Santiago, a distance of approximately 280 km(Portuguese route). I want to buy a suitable backpack. My gear is generally lightweight because I travel on bike as light as possible. In the discussions of the Camino de Santiago people say the ideal backpack is 30-35L. Living in a hot country, I always get a wet back from my backpack in the summer, and I really don't like it, so I started looking at backpacks with mesh. Of the options I looked at, I liked the Osprey Exos 38 and Decathlon MH500 38L. In the ultralight range, I like the Zpacks Arc Haul 40 and Grossamer Kumo 36L (but it doesn't have mesh).