I think this is an EXT framework issue. Having worked with ext in the past I know in 3.x and lower (Aware currently uses 3) all summing is client side and there for it is nearly worthless for a data set of more than a hand full of records. There would need to be a server side sum applied prior to pagination to handle results greater the than the number of items on your page however...
Even if that was fixed, the EXTJS grouping does not (and has not ever) worked well, There are two reasons for this 1. it's client side so same issue as above. 2. It's just plain and simple wonky, I have seen the exact same value split into 3 groups on a single page!!
At the end of the day the only practical way to deliver 'true power' from features like sum, filter, group, etc. is to have them calculated server side then presented to the client.
