AIM strongly recommend VP Performance Kits to upgrade '98 and later Harley-Davidson stock clutch for any performance engines, even a Screamin Eagle Stage 1 Kit (Exhaust pipe, Air cleaner, EFI tuner). Because as you know Harley-Davidson has reduced clutch lever effort from 2006 Big Twin model by reducing clutch spring pressure about 10%. Even though clutch performance is 10% less than before, the current engine size has been increase to 96ci. That means clutch performance may not be enough when you do any engine performance modifications.
VP Performance Kits are the best product to upgrade Harley-Davidson stock clutch and eliminate unnecessary heavy clutch pull (not like a regular heavy duty spring), but add up to 40% additional pressure to clutch as engine RPM increases (Centrifugal Force) to make very strong clutch clamping force. VP Performance Kit is including VP92T and Clutch spring.
Note: In the Chart, The clutch will not slip when there is your engine torque curve below the clutch pressure curve. These pressure and handling torque in the chart are calculated with stock primary and final gear ratio. These numbers will be change by primary and final gear ratio.