Been to the high fifties a couple of times (most certainly a 'guest appearance' rather than sustained), in the nanosecond that I looked away from the engine instruments, I did not notice the curvature of the earth. Been to 48 a lot, a more determined search did not reveal any sign of curvature. I think a lot of the 'curved earth' perception comes from photos - I take loads of photos and (because the lens is wide angle?) the earth appears curved - makes for a nice photo, but it's not what I saw with my Mk1 eyeball.
Ray