Jasper,
With your patch, gradient rendering indeed seems to be better. But it's also much slower. Create a file with 1600 semi-transparent overlapping circular gradients using clone tiler, then run:
time inkscape file.svg
and press Ctrl+Q while it's being rendered (try to press it at the same moment each time). On exit, time will show you the time spent for the run, including both loading time and rendering time. Without your patch, it takes on average 28.6 seconds on my system; with your patch, 42 seconds. I think the difference is too big for the patch to be acceptable as is.
I think it's fixable, however. I didn't look into your patch in detail, but it doesn't seem to introduce any new math, just rearranging what we've been doing before. So perhaps by tweaking it, removing nested calls (as e.g. in NR_COMPOSENPP_1111 which has two layers of nesting!) you will be able to speed it up significantly without compromising quality.
So I would like to encourage you to work further on your patch, which is will be very welcome if it can be made as fast as the current code (or maybe even faster, who knows).
Thanks!