Moved the Stretch/Sharpen/Unsharp mask options onto the tab with the output text and – get this – it now works in real time. I.e. uncheck the Sharpen option, and you’ll instantly see the output image without sharpening applied. It’s pretty sexy.
Also, playing around with the unsharp mask and blurring multiple times seems to improve the output a lot. I’m still trying to work out just how many times is best (maybe have that as an option…?)