They try to make the "blob" a single crystal with no stacking faults. You will get some errors in the crystal structure, but it will not be predictable where exactly you get those errors.
Making a single crystal that big is not a simple task. Natural single crystals are on the order of fractions of a cubic millimeter, while the single crystals they make for semiconductors are greater than 12" in diameter and several feet long.
My guess is that the variations in processors are probably not going to be primarily due to the base silicon substrate, but on the hundreds of steps of processing that happen beyond the step of making the single crystal silicon. Photolithography defects or distortion, doping issues, unfavorable tolerance stackups, etc... There may be some issues due to the quality of the silicon, but my guess is that on a pie chart of issues, it would be more like go/no go issues than slight variation type of issues. Just a guess though, I work in disk manufacturing, not semiconductor manufacturing.