Yeah, it ... takes more fiddling-with than a PC-focussed scaler, but the results seem pretty nice. I've been using it with my Q950's onboard video and it's been rock-solid. The only thing I've had issues with with it so far are some of the weird older modes on my Acorns that are very low-resolution and have other weird and TV-like characteristics. The documentation does have a table of PC modes and suggested settings for them, which is a good starting point.
I had everything half put away when I went back and read your instructions. Set everything up again and used the 'TOT PIXEL PHS' of 1456 you suggested and got to almost perfect, from this IIsi card no less! This is a vast improvement over the seat of the pants approach got me above. Desktop pattern is perfect with zero banding.
You're undoubtedly right about the inputs being video feed specific. There appear to also be adjustments for widths of front and back porches. With the proper scanline width set and properly divided as front porch + active area + back porch etc, I should be able to get the rounded corners set up correctly and hopefully fix the very slight tearout at the top.
This should work out very nicely for my SE/Radius TPD if this IIsi/SuperMac Spec/8 SI setup is any indication. We can probably nail down Portrait and Pivot scaling as well.
One day I'll read the docs, but not until I'm feeling better, playtime was just about right for today.