I only have experience with a Ti Parts Ti Tensioner Wheel for Brompton. At 6x the price of OEM it performed miserably, making the bike dysfunctional, and had to be taken off. Phil Wood BB cups provide no sealing. Good luck there!
Yikes! Ok, maybe I should just look at the Shimano one... I think it's the UN55, correct? Does anyone know where I can find the one that fits the Brompton?