I've been experimenting with procoder (1.5 & 2.0) versus TMPGenc Plus 2.5 and thought I'd share some observations with regard to GOP structure and scene changes.

"Closed GOP" was left unchecked for all tests.

Procoder 1.5 and 2.0
------------------------
"Automatic" GOP seems to work well - in fact it just does the same as setting GOP structure to "3 frame IBBPBBP..."

Scene Change detect.
--------------------------
Although I read on this forum that TMPGenc doesnt work properly in this regard, I found it appears to work flawlessly when "scene change" checkbox is ticked. There's no need to use "force picture setting" unless you want to manually mark a specific frame.

On "highest" and "mastering" settings Procoder 1.5 and 2.0 do automatically detect scene changes BUT they both missed a scene change where a TV studio shot changed (switched cameras from long shot to close up). TMPGenc didnt missed this change. It appears that Procoder is not as sensitive to scene change as TMPGenc.

Complex Movement
------------------------
On "highest" and "mastering" settings (other settings not tested) both versions of procoder look for complex, rapid movement and insert several closed GOP's with multiple I and P pictures where needed.
TMPGenc on "motion estimate-FAST" didn't adjust the GOP's in this way for the same scene.