On my last 10 day self supported tour I had about 3200 cubic inches of pannier space and they were nowhere near full. I am sure that I could have jammed everything into the rear panniers which are about the size you have. The exception is my tent which always rides on top of the rear rack. Space will vary widely depending on what items you take and how small they pack. For example a sleeping pad can pack the size of a water bottle or be a really big item depending on which one you choose. Other items will vary similarly.

You don't mention whether you are camping and cooking. That is another factor.

In any case I don't think 2300 cubic inch panniers are overkill unless going extremely compact and ultra light or "motel-ing" it. After all you don't have to fill them.