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Since P90s are already single coils and are naturally brighter than humbuckers, would an aluminum bridge and tailpiece be overkill? I play a lot or rock and stuff most people would call punk.

I don't see aluminum giving you more brightness...it should give you more clarity and sustain. Tonally, aluminum is more warm than bright.

Aluminium is less dense than steel and brass is denser again. The denser the material the more the sustain and the brighter the sound as it keeps more of the upper harmonics. Aluminium would give a more 'woody' tone just be sure that the alloy is suitable as some are soft and will wear out/ groove quickly. There is an alloy that has is as strong as mild steel, T7075 and it is anodised a puke green/yellow and is usually used as nut's and bolt's in aerospace and motorsport. They cost a bomb!
Also check out T5051 T5 and T6061 T6 as it is more available and can be anodised it all sorts of colours, any want a bright blue or red bridge?

P-90s (or humbucker sized clones) are not at all brighter than humbuckers, just because they are single coils. Their usually higher impedance due to more windings compensates that quite well, and the type of magnet also plays an important part. For example (quot;Oh no, here he goes again...quot;) the Gibson P-94 neck pickup I'm using in my Tokai Love Rock as a reference for several bridge humbuckers sounds very warm AND clear at the same time with lots of harmonic overtones which fits that one piece mahogany/solid maple top guitar perfectly. I also reinstalled the lightweight aluminum tailpiece that came with this LS 120 model which breathed new life into it (had a chrome plated heavy one there for years).

Most bridge PAF humbuckers (that I have access to at the moment) sound brighter/thinner than that and have a hard time keeping up with these P-90 characteristics which also show up with the P-94 bridge pickup that has a ~500 Ohm higher impedance.

So to sum it up: lightweight tailpiece and P-90 will go together very well in my opinion.

You can't go wrong with an aluminum bridge no matter what pickups you are using.