The highest frequency the chip can output is 3906Hz, which is the equivalent of changing the video output every four rasterlines. Similar with ADSR, for which the smallest possible configurable attack time is 2 ms, so about 32 rasterlines. But thankfully, there's also circuitry that is wired more directly and and gives instant output: the volume register ($d418)! This has been used in the past to generate samples at high frequencies (16+ khz), and is a good choice for generating video, as well.
Talking about GCR decoding, I found it tedious to handcode the decoding tables, so we instead wrote a solver that generates them given the code that uses the tables, and the desired input/output.
Some monitors really don't like to be syncless for extended periods of time.
If anything, I'd be scared of breaking the SID if hot plugging the audio output into a monitor.
@Jammer: Would be interesting if you did hotplugging or turned off power before you toasted your hardeare
