At the top of each thread page display, you show a chart with the following headings:
Participants (17) --- Posts (100) --- Answered/Replies --- Members' Posts Only
In the "Members' Posts Only" column you include the word "Fetch" as a link in order to show only that member's posts for the currently viewed thread.
You could do the following...
Only show these headings:
Participants (17) --- Posts (100) --- Answered/Replies
The "Posts" column already includes the number of posts just for that user... you could turn this number into the link to all posts from that user (replacing the "Fetch" link, and deleting that entire column).
Also, the number right after "Posts" (in this case -> 100)... could be a link to revert the thread view back to "Normal Thread Display", showing all posts instead of just a particular user's.
This would reduce bulkage and be intuitively user-friendly (keeping the hover-text would, of course, be even more helpful).
The number already represents how many posts are from that particular user... a link would intuitively lead one to be able to view just those posts from that user.
This would also add an intuitive place to change the thread view back to normal. Instead of having to know that such a link is also under each person's name/avatar on the left portion of the screen in the main thread-viewing area.
Your friendly neighbourhood back-seat programmer,
Stile