I feel like you need this calculator...
Next to the fx-85GT which I think is similar to your 8200AU. They are certainly based on the same calculating platform. The Casio's latest (cw) platform is very high precision, I think 20+ digits, and the Sharp is 16, not that anyone needs more than 10 though. The display on the Casio is better - higher resolution and slightly better contrast, but the Sharp is still good and sufficient for needs. The Sharp is far more direct in use - the 'change' button just cycles through output types. It will convert straight to other bases or Deg/Rad/Grad, and change the mode at the same time. Memory handling seems to me to be WAY better (i.e. what way are you meant to store a result into variable 'A' on the Casio? - you think it should be the variable button, but then its a menu select for the variable, another menu select for what you want to do with it, and even then it isn't obvious how to store your result!). The Sharp is just STO A or STO B etc. Also 1÷2E3 gives the expected answer.
I will post you one from the UK if you can't get one there!
Yes, I think the processor must be slower, and as a result, the integration is more limited. They also have different methods of differentiation, and maybe the Casio one is superior.
I don't have a 991-CW, but do you know what it can do with the complex functions? My experience of older Casio 991s is that complex is limited to arithmetic operations only. This seems to be the case on the W506T - i.e. it won't do Log or Sin of a complex number. But then many 'scientific/non-graphing' calculators (except for HP) don't.