It sounds like 90 lines to me. The downside I can think of is that the chroma edges can get blurred, resulting in reduced sharpness. But chroma sharpness is not nearly as easy to distinguish as luma - and rainbowing is much easier to spot.

On pre-SuperNTSC disks, there isn't much I information past ~105 lines or Q information above 64. SuperNTSC disks can have up to 2mhz/160 lines.

In theory one could have ~360 lines of chroma but it would become extremely difficult to figure out what luma should be - a 3D filter can do it if there's no movement, but that's about it!