the Weicher's aluminium brace is certainly light, and i have one on my GT. however, it is not what you'd call 'fixed'. there are 2 bolts either end (ie: it's 3-piece) allowing the pieces to pivot. plus there's a reasonable amount of 'slop' at these joins. basically, i don't think it's very effective cos there's so much movement. it is a major reason i changed it.
i now have a MOMO PitStop version (may be hard to find now). whilst it is also 3-piece, it is solid when assembled, with not significant movement/pivoting.
http://pic20.picturetrail.com/VOL97/467659/18994705/361369534.jpg
http://pic20.picturetrail.com/VOL97/467659/18994705/361508279.jpg
the more usual red MOMO versions (made as an official Alfa part) is a genuine 1-piece solid brace, also good IMO.
i suspect one major reason some braces are multipiece is for manufacturing and packaging purposes. not functional. there really isn't much point in them being 'adjustable' IMO (sure, you can apply a little bit of tension to push 'outwards' agains the suspension bolts but i doubt that makes much difference).
i continue to think a strut brace is a good idea. minimising chassis-flex not only improve suspension performance, the reduction in chassis-flex should help prevent your chassis from softening over time. but again, how significant this is...i have no proof or quantitative measurement.
from alfisti.net, perhaps either the OMP or Novitec like above:
Alfa Romeo Shop - Tuning, Styling, Fanartikel und Modellautos - Alfa 147 omp Stahl-Domstrebe omp Domstrebe 48304 / 48309
Alfa Romeo Shop - Tuning, Styling, Fanartikel und Modellautos
this Biscione perhaps looks the best IMO, but it's listed for the 156. i'm unsure why/how it wouldn't fit a 147 (i could obviously be wrong in thinking their front suspension layout/strut towers are the same), so perhaps check with alfisti.net:
Alfa Romeo Shop - Tuning, Styling, Fanartikel und Modellautos - Alfa 156 omp Stahl-Domstrebe omp Domstrebe 48304