Yeah speed readings don't differ much going from 215/40 to a 215/45. However, there are noticeable trade-offs in terms of handling and acceleration. I had 215/40 tyres, and subsequently went to 215/45 tyres. I found more sidewall flex on the 45 profile tyres and a slight drop in acceleration, probably due to a larger diameter, increased tyre weight, or both. That said however, it probably is neglible in most applications.

To the best of my knowledge, tyres in 215/40-17 that do meet the load requirements of a GC8 include Falken GRB FK451 and Continental ContiSportContact (sic?).