So the Foster driver is regarded for its broad frequency response. That can be good and bad.
The highs can be harsh to put it mildly. What the Lawton mod does is that it brings a more neutral response to the headphone.
It’s done in three part; a “TuneUp” kit that addresses resonances behind the driver (basically acoustic foam on sticky tape), the pads used (but many use ZMF pad because, well they’re ZMF pads, which also affect FR), and lastly, the Lawton Chambers (cups).
The chambers are the most expensive component of the mod. Mark Lawton uses a mix of exotic and more traditional “tone woods”. They’re much larger than the standard cups than that of any of the Foster driver headphone. Remembering that the woods are really the only difference between Foster driver headphones, this in theory would make the largest difference in sound quality.
He has a whole “Tone Wheel” selector on his website.
https://www.lawtonaudio.com/tonewheel.html
I went with Marbled East Indian Rosewood (Type 4), which is still in the curing process (it’s the last pair available from this particular wood blank).
@PapaEmeritus has completed a few full Lawton Mods. He’d be the one to ask regarding their sonic qualities.