The height of speed-humps in the UK varies, some are nice to go over and some are horrible even at slow speed. Lowing the height of a vehicle in the UK may not be a good idea. Do they have speed-humps in the US? < not ridden there and do not often go there so not sure
I like the cost of this mod (FREE!!!). I wouldn't do it on my PCX though, the suspension already bottoms out for me over some bumps. I can't imagine how easy it would be to bottom out with less spring. My city has a lot of potholes and I do a lot of curb hopping, so I can imagine this mod would work well for someone in a city with well-maintained roads.
Did you notice reduced cornering clearance? I removed my centerstand for more cornering clearance and to prevent the infamous crankcase break that only happens on the 2011 model, but I still rub my plastics from time to time. This would bring you closer to the ground, meaning less potential lean angle.