Variable speed drives are a great investment in most pumps of any size, at the initial installation stage, since the extra capital cost is lower than retrofits. Unfortunately, most project managers are more oriented to low capital costs than to low long-tem costs. They are gone by the time the plant is in production.
Most discussions and analysis evaluate power consumption by properly sized pumps. However, in my experience, about 80% of pumps are seriously oversized for normal service. Some times there is a reason for a large margin, but usually it is just because engineers know that if it does not work they are in trouble, whereas if it wastes energy nobody notices.
You have mentioned pumps oversized by 20%. I have seen MANY oversized by much more than that. Worst was over 500%, where we replaced a 100 HP pump with a 20 HP motor, slower speed and trimmed impeller. It still ran with the control valve half open, but at least the noise level dropped and the valve did not require regular replacement.
All mills should survey control valve normal positions and analyse pumps operating well below full-open, with a view to variable speed conversion.
A supplementary benefit of variable speed pump drives is a great reduction in nose level in the mill.
Many agitators could benefit from variable speed drives too. The lack of reliable design criteria for most agitator service leads to overdesign in many cases. When you vista several mills, it is striking how much agitator size varies for equivalent service. clearly the larger ones are too big.