It’s actually rare that scaling adds any input lag, most TVs have the same lag regardless of 480p,720p or 1080p. There are exceptions, but it’s not all that common. The main exception to this is, of course, interlace resolutions which often add 1-3 frames or more of lag onto your TVs base time.
I would definitely mod your Xbox to output 480p, interlace should be avoided on HDTVs wherever possible, that’s true if you’re using OSSC or any other scaler (or direct to TV). Particularly with OSSC you have to put up with some flicker if you want lag free deinterlacing.