I've got two questions about the rear control arms (upper and lower):
1. Are the two arms the same size (i.e. interchangeable)
2. What's the weight difference between the stamped steel ones and the aluminum (?) ones on the 05+ cars?

I'm trying to decide if it's worth swapping the upper (steel) one with an aluminum lower when I change out my lower control arm with an adjustable unit.

 

1. The upper and lower on the R53 are the same. You can, for example, buy two pairs of the same arms, and install them on the top and botom, interchangeably.
2. I don't know the weight difference between the two.

Your value question depends on your needs and wants. IMO, unless you are competing, the weight difference is not worth the expense. Adjustability in the lower arms allows camber changes needed to alter handling characteristics. In addition, weight reductions toward the rear of the car (all other things being equal) just move the weight bias further forward on a car that already is biased toward the front.

Various aftermarket units weigh in differently. I think that someone did a comparison of major brands in a NAM thread not too long ago(?) I'll see if I can find it.

 

FYI, my car is a 2004, MCS, daily driver in California (few worries in the winter) I have H-Sport lowers installed and I'm running OEM uppers at this time, with 19mm Alta adjustable rr anti-sway on "stiffest" setting, OEM "Sport" springs and Koni FSDs with SPC camber plates in the front.

Here is the thread I remembered in which some lengthy discussions occurred. FWIW...

Enjoy.

https://www.northamericanmotoring.co...with-pics.html

 

I'm starting to think replacing the upper arms probably isn't worth it to me. I'd like to reduce the unsprung mass to improve ride quality, though after thought I doubt reducing unsprung weight by probably 1 lb wouldn't improve the ride enough to justify the cost.

My car's an '04 MC with:
-Alta 19mm rear sway bar
-Stock MC front sway bar (SS, not the SS+)
-Helix camber plates
-H-Sport LCAs (sitting in my garage; installing them next week or the week after, whenever I get time)
-[coming soon] TSW springs with Koni FSD shocks

It's mostly daily driven with the occasional (6-12 / year) autocross / HPDE

 

^^^ I see you are looking at the TSW springs. You will want the TSW lower arms too. They will give you the adjustment you need to bring the camber where you want it since there is no adjustment in an 04 MINI.

You may also want a set (4) adjustable end links for the sway bars this will allow you to corner bal the MINI and take full advantage of your current setup and future ones too.