The lightest possible +20 mm disc rotor adatper are the "stacked washer" type, photo below.

Is there any reason why this is not as good as the one piece adapters, which have a brace between the two bolt holes?

No reason. Except maybe the greater chance of installation error, but that's all I can think of. They've been coming on bikes from the factory with them for a couple years, never a problem.

The bracing provided by the one piece is useful. IF you don't care about ultimate brake performance then the other is obviously lighter.

_________________
For certain parts stiffer is more important than lighter.

If were talking about a post mount installation, if the mounts are properly faced, is there really a need for the cps conical spacer hardware as opposed to an alloy or ti spacer of suitable length?

_________________
Michael - The Anaerobic Threshold is neither...

Assuming the calipers are also properly faced, and not sloppily made because they are relying on their CPS washers, then sure.
If you're talking about the CPS washers in the top picture, those serve a different purpose, they square up the bolt heads since the two post extensions are different lengths.

Even with post mounts these calipers like as much stiffness as possible. Any torque in the mount can alter the braking feel.
IMHO the linked adapters are all made that way for a reason. Doesn't mean you can't experiment tho.

BTW there was a ForSale ad on here for some carbon post adapters, looked wicked light.

I noticed that on the formula adapter, as well as the shimano in the photo, that the "posts" are of different length. This has to be for a reason, I would guess this positions the calliper and subsequently the brake pads in a more optimap position.

My feel is the the brace helps strengthen the mount.

I am running my post mount callipers without the CPS washers without any alignmenbt issues.

re: the last sentence, when you omit those washers, don't you still have to use somekind of washers to maintain the correct height for the caliper?
otherwise the pads might contact the rotor too low on the rotor surface?

The mounts would have to have the same offset on each end as the bracket does. On one bike, I even had to put an extra washer because they had apparently made the post a little short and the pads were running a little inside the swept area on the rotor. I swapped the bracket with another bike and same problem so I decided they must have milled the post down a little too much at the factory when they made this particular frame. This is something I now check when building a new bike. Make sure the top of the pads are parallel to and dead even with the top of the rotor.

_________________
For certain parts stiffer is more important than lighter.

I refer to the CPS washer that sit between the calliper and bolt head - e.g. Avid Elixir have them.

You cannot remove the CPS washers that sit between calliper and adapter as the disc will contact the calliper.

Have used these with a BB7 caliper on a 180mm aro-8 rotor.
I had to align the caliper a bit higher with small washers, different ones too, as I had to align the pads concentric with the rotor.
Felt no difference in braking performance.
It helped to reduce the weight to 253g (frontcaliper + I-link shiftcable + Steinbach levers + all bolts titanium (including those on the caliper) plus the adapter, without disc).