Imagine multiple tearing lines on the same screen; that's exactly what I get, so it's all juddery and jumpy. We're talking an intel G41 Express on top of WinXP, everything up to date.
Are you certain your drivers are recognised and working, and that the Hardware manager shows no malfunctioning devices?
DOSBox ECE r4035 runs Monster Bash smoothly on my PC using surfacepp in fullscreen mode. It is an AMD A4-3400 APU with Radeon (onboard) graphics, under Windows XP, and my display resolution is 1680x1050, so the game runs at 1280x1000. I will see if I can add some logging to check what is going on on your side, but right now I should blame either SDL or your configuration. I don't have low-level control on how SDL puts the image to the screen, so it may be that SDL somehow dislikes your software-hardware configuration.
Now, there's this source port called "Chocolate Doom" that tries to do something similar to what this patch does (similar as far as the intended end result is concerned, of course) and I don't get this type of tearing at all, meaning the "more work to do" part of the equation could still potentially be improved.
Well, I don't get any tearing with my patch either, and by more work to do I meant that high load aggravates the bug or problem, which must be there in the first place. When my patch begins to stutter on my PC, it does so gracefully and without many tearing lines.
It's being done with SDL2, though, so I'm not getting my hopes up
That's a different kettle of cod. It has, for example, native support for hardware-accelerated integer scaling with independent horizontal and vertical scaling factors—just what the pixel-perfect mode needs!