I think all brands fudge on bin size.
we have r65 that is supposed to hold 300 and most corn you can get in is about 275bu, however in wet milo that is peaked high you can get about 310, beans about 250
The front and rear extension pieces are 8" shorter than the 60/7088 extensions, side covers are the same size. You can add that back without a problem, this is on the power folding bin extension. Checked on the bigger pieces for our 5088, very cost prohibitive (over $2k in parts). You can easily make this yourself, just find a 60/7088 to copy. Frame, final drives etc. are all the same, can easily handle the extra weight.