Should I buy a Canon 100-400 L?

[table1349]

Ok, but is this difference in sharpness clearly visible also with lower end cameras like the 450D (compared to the 1D)?

Lenses produce sharpness, or lack there of, not bodies. The body records on the sensor what the lens projects. The advantage of an upper end body are the added features for shooting. Faster fps, wider ISO range, better control of noise at high iso, more focusing points, more metering modes, etc.