It looks like it could easly compete with the Omni / SGM2 in term of capacity .
The caracteristic you put for the Phoenix, are the one coming directly from the F18A? Or it is something additional?
Does a colecovision + F18A can display 256x240 ?
The Phoenix is using the F18A core for the VDP.
Yes, the CV+F18A can display 256x240, or basically 32x30 tiles.
Not 100% sure, but I believe it offers those kind of extra resolutions, in addition to a wide color palette to choose from. I've seen some homebrewers like digress display NES-style sprites with the F18A, but I don't know if they're 3-color sprites or just overlapped 1-color sprites.
The F18A allows more colors per pixel (2, 4, or for tiles and sprites from a palette of 4069 (12-bit) colors, among many other enhancements like horizontal and vertical hardware scrolling, two tile layers, independent tile horizontal/vertical flip, sprite priority, and palette select, 32-sprites per scan line, etc. There are 64 12-bit palette registers, so without scan line or GPU tricks you can get 64 simultaneous on-screen colors from the 4096 possible.
You should check out the js99er (http://js99er.net/) emulator for a quick way to see a lot of F18A-specific demos that show some of the enhanced capabilities.