Setting the resolution etc. will obviously change everything on the screen. For a non-fullscreen application like mIRC (maximised is not the same as fullscreen) it'd make no sense for it to do that - more specifically, it'd be very, very annoying. As for speed issues, well yes a lower bitdepth and resolution is less taxing on the graphics card. But mIRC isn't particularly graphics intensive anyway, so if it's causing issues the user should probably be using different screen settings in general.
As for resolution-specific scripts, tell the scripter to change the code. There's very little in any mIRC script I can think of that can't scale to any resolution if coded correctly, and absolutely nothing that can't handle 99% of situations and fail gracefully on the remaining 1%.
