Seems some lowering spring companies recommend trimming (Eibach, Tein, some H&R) while others are silent (UP, T-Sportline, and others). I have Eibachs and the instructions say to trim 25mm in back and 20mm in front. Did Eibach include this instruction when it's actually not necessary? Why do they give a different spec for front and back? All BS? Maybe - but I assume a reputable suspension company would add that into the instructions for a reason. If you look at some Eibach instructions, different trimming amounts are recommended for different cars and even whether you trim the top or the bottom. Again, I don't believe it's random.
Here's a good write up from the Tein website:
So, in regards to trimming bump stops- why is this important? If we find that aspects, other than damper stroke length, seem to be very generous, we know that we can make a lowering spring that will work with a shortened bump stop. So we include a chart by vehicle application that shows how much you will need to trim for front and rear when installing our lowering springs.
Some cases may require a different type of trimming style for the bump stop. Typically it is just trimming the bottom portion of bump stop and retaining the top piece (A Type). Sometimes it’s the reverse (B Type). In other cases, you may need to trim a middle section and retain the top and bottom piece (C or D Type). See below the different style cuts we recommend, depending on the vehicle:
We know several customers skip the bump stop trimming step. This can cause several issues. We’ve added this measure so that you can retain a bit more stroke length at the newly lowered ride height. This also helps improve ride quality. Because the bump stop is made of a high durometer polyurethane material, it is like a secondary spring, but a really stiff secondary spring. Since the dampers are not valved to handle such a stiff secondary spring, it can make rebound feel really harsh. Some may say bouncy. Constantly hitting the bump stop can cause the damper to prematurely wear or potentially blow out, too.
At the end of the day it's up to you on what your comfort level is. I'm sure based on driving styles and road conditions many will never run into a situation that maxes out the stroke to hit the untrimmed bumpstop so in those cases trimming or not will not matter. Note that Tein says not trimming "
can cause several issues" rather than
will cause several issues - meaning you may never experience any problem if you don't trim them. What ever amount your spring lowers your car by is the same amount you will lose in damper stroke.
Best bet is to follow what the engineer states to do. Less chance of a problem and if you have a warranty issue it's one less argument that can be used against you. Only thing lost is a bit of extra effort.