Hydraulics can rev to 8300rpm but it really is a benefit to go to solids beyond 7500rpm. The problem arises because not only do you have increasing back pressure at high rpms weakening the effective valve spring pressure, but of course the hydraulic follower itself is actually exerting an opening pressure (caused by oil pressure) again weakening the effective valve spring pressure. Lastly you also have the weight of the oil filled hydraulic follower, which all has a significant negative effective on the valve train at high rpm.