Sharp filter rejection rolloff is always accompanied by large group delay variation. When both fast rolloff and flat group delay are needed, equalizer can be used. Sometimes multiple equalizers are needed to achieve very flat group delay performance.
Send your requirements to us when you need both sharp rolloff and small group delay variation. Delay equalizers can be added to almost any filter to flatten group delay response by up to a 20:1 ratio, at the expense of insertion loss.