Correct me if i'm wrong, but how can his mates still see him? There is no mention of letting others willingly perceive you.
Congratulations, your magician is now a non-existing person, just like in 1984.
He's the perfect assassin! Until he runs into some stupid traps which get triggered by him stumbling over some whires.
Also, i don't think that the Inquisition can afford itself to be stupid/weak. I dare to say that most magical active Inquisitors use all those nice detection spells (and of course MR-boosts) to find guys like said mage.
Wait, what other factions are also interested in Mages and their supernatural doings?
There's Wissenschaft, Tol Rauko, Samael, the Hand of the Emperor, also the Knights of the Heavens, the Azure Arbiters, Les Jaeger, the Order of Miguel (your Free-time Church Fanatics), the Magus Order, the Order of Yeduhah ... i nearly forgot Black Sun (you sure your body parts can't be used in our next experiment?) ;-P
Sure, 1000 Inquisitors are few, but all the other groups are nicely adding up, and then there are of course still the lone individuals with their own Agendas.
You are not the only Mage out there with no ties to these groups. Some may like you, others simply want to abuse you, but you sure are NOT alone.
Bat to the topic: Magic is not underpowered in my opinion, since it is far more flexible than Ki Techniques or Mental Powers.