I would do that with my old CR9080, and it would have done more with a 35ft header. If a Lexion 770 will not do better than that then it can go back from where it came from. I always knew the Claas marketing machine was good, I thought I could see through it - we shall see. And my average field size would be less than yours.
If I change my spreadsheet to 200 hrs/10acre an hour costs go up to £25.10 per acre. (plus extra corn cart costs).
Field size, shape, crop, yield, tolerance to losses, stubble length, obstacles in the field.... I find it best to look at weekly or seasonal work rates achieved, not spot or daily.