Mapline, I have not noticed and major affect of them being lower. When reversing toward a kerb I think the lower sensors see the kerb well. The sensors need a reflection to detect an object so gravel can some times give a beep. If reversing from level up a steep hill I might get a beep but I imagine the VW version would give the same. The kit I used specified a minimum height for the sensors from the road and my locatyion is above that so no issues really. I didn't need any type of spacers the kit sensors are numbered for particular locations L1,M1,M2,R1 and are shaped so that when pushed in to the bumper they angle correctly.
If you have any questions drop me a line.
Dave N