Still, I am a bit baffled by the situation. On my triple setup I can measure about 25mm gap between the inner side of the right crank and the top part of the derailleur and also a Q factor of about 160mm. If I were to correct these values for the ring spacing when going down from triple to double crankset, by about 8mm, this would yield a 17mm gap and 152mm Q factor. Now the Q factor of the particular Ultegra crankset you are using is 146mm, from on-line specs. Dividing 152-146=6mm by 2, this could explain a drop in the gap of 3mm, but not the gap between the driveside crank and top part of the derailleur turning to negative values! Spacers, on their own, can help changing extension to left or right by a few mm but not centimeters. Something is grossly off here, in which the spacers will not help. Is the power meter taking so much space on the left hand side that it will be never possible to have enough space on the right for the derailleur? If this were the case, the project would need to be abandoned, at least in the current form.