Would a maxed-out unit of Knightly Orders be a viable hammer unit in T9A? That'd be 690 points for a 12-man unit with a full command group. Another 30 pts for a War Standard would be a straightforward choice. Such a unit would put out a whopping 19 lance attacks with S6 WS4 on the charge. Charge being the keyword, so it would be more meant to land a decisive killing blow than slog it out through protracted combat. Not that it'd be helpless in protracted combat either, having all those attacks at S4 would still be quite significant, and it'd resist incoming damage mightily with its grand 1+ armour save.
Conventional wisdom back in 6e/7e day was that a second rank on knights is an expensive extravagance not worth it. In T9A though, that second rank does add some killing power, and perhaps more importantly, lets the unit preserve its killing power in the face of casualties much better than a single-rank unit. Also, having two full ranks (and a guy to spare) gives the ability to negate steadfastness, if that should become relevant.
The main drawback I could imagine, just theoryhammering here, is that you'd be rather reliant on getting the charge, rather than being charged, which might not be easy when there's other cavalry in the world with faster horses.