If the point is to solo tank arbanus and guyass as DT, it's fully possible. Does that measure if the class is OP or not, maybe? Does it matter? No. The soldier trio works pretty good as is today..
Would I like the DT changed as OP wants it, no, not really. The spell coefficients are low (and based on magic dmg) for a reason, upping those even a bit (or changing them to be based on str/cbr) would make the class extremely OP, and that for no reason at all. How it is today (and has always been) works good and makes the class really diverse, just like everyone likes it