Done: http://repo.xposed.info/module/com.mohammadag.screenshotdelayremover
/r/xposed post: http://www.reddit.com/r/xposed/comments/23z2jo/remove_screenshot_delay/
Ninja edit: no idea if it'll work on 4.0 and 4.1, the code is the same for 4.2+, but for 4.1- I did some blind coding.
There's a module for that. Also, should work on 4.3-4.4.x devices.
Edit: http://repo.xposed.info/module/pl.suzume.xposed.xperiamultimini
Sony has enabled it on the Xperia Tablet Z2 which runs the latest version of KitKat.