I made the following change:
<key>Threshold_Low</key> <array> <integer>0</integer> <integer>97</integer> <integer>97</integer> <integer>98</integer> </array>
My Cinebench R15 score when up to 93. The stuttering in Heaven is gone.
While typing this, my GPU core reads 50MHz, but after I navigate some web sites, the core goes to 405MHz and sticks there for quite a while. I may try 0,95,95,97 next.
UPDATE: 0,95,95,97: GPU core doesn't idle at 405MHz for near as long. Cinebench score hovering around 90. No stuttering in Heaven. I think I'll keep these settings for a while.