Howdy All:
It seems to me that the "Jury is still out" on the port dividers only because some haven't taken the time to properly fit and weld in the divider. When not properly welded in they can come loose and rattle. Clifford used to claim a 15% increase by adding the divider. That is very, VERY optimistic. My guess is more like a 2 to 4 % increase with a log manifold and 7 to 10% increase when using a header. The increase is mostly likely higher at higher rpms.
IF the divider is properly fitted and welded in in three spots as recommended, having it come loose is quite unlikely.
Here are the hard facts; parts are expensive. A port divider is $20 last time I checked. SI valves, 1.6:1 rocker arms, Valve springs, etc.. are more expensive, and no one questions the performance increase potential on these items, yet no one, TMK, has gone to the trouble, expense and process to individually install and quantify the effect of each of these separately. My point is no one can tell you, for sure, what benefit will come from what, but we all know that they work.
So, buying a port divider, properly fitting and welding it in at the time the head is down for machining is IMHO a no-brainer. Does it have any Coolness factor? No! It's just a grungy piece of iron that no one sees, but in your heart you know it's there and doing it's job on making a poor exhaust port layout just a little better.
That's my story and I'm sticking to it.
Adios, David