The time spent "improving" in the terms of the project is time not spent improving in areas that matter to you (or your employer) more. If the fix works and the itch is scratched, extra time spent developing specialized knowledge about that project's build/test systems or standards or personalities for is likely not time well spent, especially if the likelihood of submitting a second patch to the same upstream is low. If you plan to keep contributing over the long term then sure, but that's not the only case.