I also have had no success with finding an expression pedal that doesn't do the same thing you describe where there is a big jump in value changes in the early part of the pedal travel. I've tried the Roland EV-5 (which is on Arturia's list), the Yamaha FC7, and the Korg EXP. None has worked any better. I have tried calibrating the input according to the instructions, have tried different settings with the minimum value dial on the side of the pedals, etc.
I also have problems using the Breath Controller input (which is one reason I bought the Lab 49 in the first place) because it doesn't have a smooth scale over the full range of that controller. It also has a very significant lag returning to Zero after I let off breath pressure on the Yamaha BC controller.
In general I have noticed that there is lag with many of the controllers on the Lab keyboard. The PB and Mod wheels can be very a slow to respond to rapid changes. I'm wondering if the MIDI CPUs are underpowered, or if the firmware/software is just sloggy.
Anyway, I'm rarely using my Lab keyboard these days (even though I like the size and the key actions, as well as many of the programming features) because I can't use the controller the way I need to with my set up. I keep hoping Arturia comes out with a really good software update for this thing. But I'm not holding my breath...controller!