do aps-c cameras benefit from f2.8 OR will f5.6 2 extra iso stops suffice

Do aps-c cameras benefit from f2.8 or will f5.6 2 extra iso stops suffice
I have a Canon 450D 18-55 f3.5-5.6 +55-250 f4-5.6

Well, as it's a chilly but Sunny day I could just experiment for myself. a 70-200 f2.8 would be nice, but it's far to heavy for my to hold since my stroke 5years ago. 250mm f5.6 iso 400 for now with 4 cats running around chasing things, or just sitting on fences. 400iso burnt out, back down to 200iso Highlight Priority Tone on Auto Lighting Optimizer on too.