My first LWFW install at around 30,000 miles (2007) did indeed introduce NVH. At 70,000 (2010) I replaced the clutch and upon reinstall, the problem was largely cured. The parts were the same - the difference was made by the installer. The first time I took it to a local Performance Shop specializing in Subaru. The second time I took it to a dealership but also all supporting parts were replaced, including throw-out bearing; tranny mounts were replaced with either new and/or STI / Group N part(s). Therefore, the quality of the install might also be due to new/improved parts not just a more experienced hand.
My setup consists of an ACT Heavy Duty Clutch and ACT LWFW for the 06-07 WRX