Managed to replace the rear speakers with a set of Infinity Reference 5010 components. Sounds much better than the standard speakers.
And using a pair of tweeter brackets supplied with the speakers, I've found a place to mount the tweeters without making too
much of a mess of the OEM look (at least, I don't think they are noticeably out of place or ott)
Currently applying layers of AG EGP for the coming months of bad weather. And after a quick wash this afternoon: