With my Comet fitted with the 'new' steering head I run a 19" front and 18" rear and the new Michelin Sport Classic Tyres, 34 psi rear and 30 psi front. Thornton on the front with standard spring boxes and a thornton coil over but without the coil and standard spring boxes at the rear.There are a couple of very seasoned riders here in Australia, who until quite recently had only just got the chance to ride their bikes with this modification done.........Even so, it wasn't until they had covered a few good miles that they realized the benefits of this mod.........Their feedback was that the steering felt much lighter and responsive, but that somehow they were readily grinding their foot pegs on the ground whilst cornering, and also the braking felt much more effective and compliant over rough roads. To me this was good feedback, as these blokes have owned Vincent's for a long time and covered many miles.........One of them actually suffered a bad crash years ago, and realized after discussions, that it was almost certainly a tank slapper that threw him off. When I was running the racer in my pic to the left, it felt amazing to ride, especially with a simple coilover shock on the front and rear........the reduction in friction was massive and the changes altered the behavior a lot....... I don't ever remember the front end acting badly even over some rough stuff and I could easily out corner many of my opponents on the track..........The chap who owns it now is a far better rider than me..........I have seen him on this same bike racing against Beau Beaton on the Horner brothers "Goodwood" bike in Victoria.......He made the exact same observations as me.
As Greg has commented, handling and braking under all conditions is a massive improvement over the standard setup and a bit like he commented - in the twisties when touring I can easily 'out ride' , aka leave behind, folk on modern so called sports bikes - they only go faster because of the power difference when there is a straight!
But having said that the major factor is vastly improved ride safety and a serious improvement in brake performance, especially when on uneven surfaces