I missed in your original post that it has a dual disc clutch. So how much do these disc's weigh compared to a stock single disc clutch? The reason you have to have a synro assembly is because all the gears are in mesh with the counter gear, and all spinning at different speeds, because every gear has a different ratio. So when you step on the clutch and shift gears the brass ring grabs the cone and slows down or speeds up the gear you are selecting, and then the teeth on the gear itself mesh with the slider which locks the gear to the mainshaft and transfers the power. So the weight and inertia the clutch disc really effects how hard the brass rings have to work. On most dual disc systems, I have messed with you do not need to use a clutch pedal to shift them, the clutch is only used to launch the car. You may want to call the company that built the clutch and just ask them if the inertia of the discs at speed is causing your problem.