I've got a 2" strut top lift right now with trailing arm spacers. But I'm planning to remove the strut tops and get a 2" lift via coilovers and I'm wondering if I can ditch the trailing arm spacers and go with adjustable trailing arms.
It really depends on how much you need to adjust the Trailing arm length. The adjustable ones only have a certain amount of threaded rod going into the arm. It does seem doable if you're going to 2" lift from coilovers.
So I have the MSI 1" correction trailing arms for sale laying around somewhere. I can measure how much shorter it is compared to a stock arm. Then I guess you can do the math going the other way. Remind me if I don't reply here in a few days.
Longer tailing arms will reposition the wheels, fore to aft, in the correct location but the pivot point is still 2" higher than it was intended from factory so your "geometry" is still off. The arc the wheel will move in is now larger so in anything but ride height position the wheel will move farther fore and aft than the stock length arm. This is exacerbated when lifting as you are now farther down from parallel in the arc. Sin, cosin stuff. On a WRX where they are much closer to parallel the impact is much less.
To keep the factory suspension geometry, which is how the points move through space, you need to get a spacer for the front mounting location.
I did a 1.375" lift in the back and got the same size spacer using stock boxed arms with poly bushings. I also boxed the front mounting bracket. You can easily jack the back of the car up from them and they aren't moving. Even with the spacer it's still tucked up into the chassis pretty well.
Same goes for the lateral arms. The best bet would be to space the rear subframe down the same amount. Even a 1" spacer would be great. But with stock arms your effective track width is now less. Longer arms again can correct this at ride height but the width change through the suspension travel will now be greater.
This is one of the reasons I only went with a 1" lift + saggy butt spacers.