wing commander did not run in interlace. it ran in 320x200 (same rez as PC VGA version). Amigas standard mode (in PAL areas) is 320x256, which is higher so that's not why it looks grainy. The reasons why it's grainy are: It's for ECS, which normal colour max is 64, VGA is 256. VGA wing commander runs in a chunky pixel format, amiga uses slower bitplains, so they used less bitplanes to speed it up. Also they ran it in 320x200, probably for ease of conversion, and to keep the speed up. Also, it's ported from the 16 colour EGA version, which is butt ugly.
The CD32 version (which also runs on the A1200) has loads more colours, and looks more like it's VGA counterpart. Although it still runs at 320x200.