Two issues not mentioned yet: if you start the display at the normal left border AND want to use the horizontal hardware scrolling feature, you will lose the 8th sprite (for each 16 pixels further left you will lose two more sprites each due to overlap with screen dma). Thats why e.g. Turrican has a slightly smaller screen to the left.
Also screen DMA (and copperlists) will take away cycles for blitter and generic chipram access, so turning off the display at an earlier vertical position will leave more cycles for blitting (with complex source combinations) and cpu chipmem access (usually starts happening with 5 bitplanes at ocs/ecs.
