Big spender, eh?
Funny, I'm rather fond of the Gielen (certainly enough so to put its limitations out of my mind while listening, although it certainly has them). I also got to know the work through the recording with Maazel and the Fischer-Dieskaus although I was pleased to hear Johnson bring a bit of weight to it that F-D doesn't quite have.
I can't honestly imagine Schäfer in it, somehow. For me some degree of bigness is needed in the soprano part. Having said that I haven't heard her in it and I wouldn't want to prejudge. Er, more than I already have.