As you and Pogue have already asserted, he was close to a goal or assit every game in his first 5 months (in fact he was involved in 50% of all our goals - way more productive than Di Maria for example) and involved in 8 in his first 13 this. So yes, I think that's perfectly acceptable for a £40m player. Especially one in an underperforming side. The fact people are losing their minds over his recent form (which has still, even by your standards, included at least 2 great performances) is precisely the point.
Worth every penny? Who knows, he's only been here a year. Satisfactory by any reasonable standard? Easily.
Comparissons with Kagawa for example, are ludicrous. Despite being more than twice as productive in less than half the time, he's even ahead of Kagawa this season at Dortmund.