Hi vonP. My first source for a great helm is Richard I's second great seal (1198). So logically examples were probably around slightly before that. By the way, whats your source on dating mittens?
In my forthcoming article on Bishop Philip of Dreux in the next edition of Medieval Warfare, I felt comfortable advising the illustrator to have a rich magnate like bishop of Beauvais in faceplate and a lightweight caparison at Arsuf. However, the majority of the Crusaders aren't depicted as greatly different from the figures produced by Conquest and the Twins - longer sleeves on the coats of mail and smaller shields notwithstanding.
I think that sukhe_bator hit it bang on the head earlier on the nature of the Cambro-Norman forces in Ireland. Of course equipment would have improved once Henry II and John got involved, but this is still before the major step change in knightly protection that seems to have come, as von.P writes, around 1200. So by the time of the battles of Lewes and Evesham it was a totally different ball game 
The Richard I seal is also where I was going with the faceplates. I figure at best 5-10 years prior you get the early development of the full face as in the 1170s one can see the beginning of the nasal spreading outward, a good example is found in the
Rolandslied des Pfaffen Konrad, I would in no way consider this a full face guard but it is a developmental step.
As for the mittens my research comes from an art history perspective. in most of the MS I have looked at I don't see mail mittens until the 1180s and same with surviving effigies (as few as they are from the 12th century). Further in MS illustration it is a bit of a flip flopping trend where many MS late leave them out. I do bow down to your greater study of martial material culture though, I only dabble.
I'm right there with you supporting sukhe_bator, the Perry figures with minor conversions (though for me they'd be quite difficult) will work for early Normans in Ireland.