Shaun, as you know I've recently fitted Teins and I guess they're still settling at the mo, but I've not had any issues with my lip on speed bumps etc (yet).
I think the main problem (if it can be called a 'problem') with a lip, certainly the Nismo style one like mine, isn't the fact of how low it is (as it isn't really much, if any lower than the OEM bumper), more the fact of how far it pokes out to the front.
If you're going up or down a slope, it'll effectively hit the slope a little bit sooner, as it's 'extended' the nose of your car.
Difficult putting in words - I hope you get what I mean!