Hot booting doesn't work anymore in new Xperias. It used to work when I had the xoeria arc. Now for your rooting wherein you want to retain the stock kernel, just use the advance stock kernel of Doomlord, it's pure stock kernel (only with recovery added). Then flash the SuperSU zip in the recovery.
Here's the link for Doom's kernel. http://forum.xda-developers.com/show....php?t=2646193
Tapatalked from my Xperia SP