Raffaele D'Amato and Graham Sumner have an excellent book entitled 'Arms And Armour Of The Imperial Roman Soldier Marius to Commodus 112bc-AD192' which contains many pictures of the above metopes with many variations of armour styles depicted. IMHO it also confirms the existance of Sarmatian/Dacian archers/auxiliaries in the Roman army and publishes pictures from Trajan's column of Danubian numeri wearing various Sarmatian/Dacian helmets, (pages162/163)
I tend to treat that book with a healthy degree of scepticism, and only take it as 'evidence' when I have other sources to back it up - D'Amato and Sumner both seem to prefer to 'live on the ragged edge' when it comes to evidence and its interpretation.
Having spent a couple of hours last week in the Museo della Civiltà Romana peering at the castings of the reliefs from Trajan's Column, I can't deny that some of the auxilia are wearing 'spangenhelms'. Ordinarily, I'd say that it's a bit of a stretch to follow that by implying that legionaries would have worn them, but one has to remember the metopes from the
Tropaeum Traiani at Adamklissi, one of which pretty definitely shows a 'spangenhelm-type' helmet (unless it's just a legionary helmet with reinforces - the sculpting isn't the best!) on what might well be a legionary (to judge by the shield):
Nice figures, by the way!