Why do different trims of the same car use different parts?

To start with, you need to understand that when designing the suspension of any particular car, engineers work out an enormous number of parameters, and these, among other things, determine the requirements for individual suspension parts.

Have you never come across a situation where, for two otherwise identical cars (same model, same make) that differ only in the engine, the manufacturer fits different suspension parts: ball joints, track rod ends, control arms, and every suspension bush at the points where these parts join? Why do you think that happens?

It is all quite simple: different engines have different weights, and so, when the weight changes, the force acting on individual suspension parts, and possibly the direction it acts in, changes too. Accordingly, the design changes as well, since it has to give maximum reliability to the part while keeping the handling and the ride comfortable, and, not unimportantly, keeping production costs to a minimum.

And it is worth noting that where in the past most car makers built quite a large safety margin into a car’s key components (the suspension among them), lately there has been a trend towards more precise engineering calculations and lower production costs, achieved precisely by cutting back on that safety margin. And this trend, sadly, considerably narrows the scope for any “garage” tuning, whether of the suspension or of the engine.