Throwing a soft spring in there may help as well. I am keen to see your results as i am tempted to try a 6boost manifold with my TD06 setup/ Looking at the std Greddy manifolds they look reasonaly well designed, and they are a lot lighter. Considering i have spent lots of money on titanium exhaust, kevlar race seats, alloy bonnet etc its a shame to go and throw 6kgs back into the car. But considering the cost of a 6boost manifold it could be a decent upgrade.
Below is a knock off copy of hte Greddy manifold, looking at it the Greddy manifolds already have a pretty good collector and countoured collector, and is a 6-2-1 manifold