I like John's suggestion about modifying the bottom summary. But there's a complication: when a more-like-this group is fully displayed, that summary line changes too. E.g., when a group with 7 additional items is expanded, the summary reports "1-35 of 35 shown." Perhaps revising the initial line to report "There are 94 matching WeBWorK problems in 28 groups" would be a stable alternative. (Detail: omit "in # groups" when no more-like-this groups are in the filtered search.)
Note: while such a group is expanded, the M for the featured item is replaced with L (Show less like this). Clicking on the L collapses the group display to show only the featured problem.