I did look into it a little, and the 2560x240p60 mode is using a 51MHz pixel clock, while the HD 5450's 2560x1600 max is right around 250MHz. So, the pixel clock alone shouldn't be a limiting factor, but I'm still wondering if there is some sort of horizontal line pixel-limit that's common among CRT Emudriver-compatible cards.
Right now, dividing 2560 pixels evenly (and multiplying the result by 0.75 to get TVL for a 4:3 screen) we can get 1920, 960, 640, 480, 384 and lower TVL test screens. If we were dividing 5120 evenly, we'd get 3840, 1920, 1280, 960, 768, 640, 548, 480, 426, 384, 349 and lower TVL test screens, all near-instantly switchable. The test screens over 1000 TVL don't really matter, but the additional screens under 1000 TVL would be really nice to have.
EDIT: Also from the TVL mod thread - A quick and cheap video showing how easy it is to change TVL screens with this approach. All I'm doing is pressing left on my keyboard.