Yeah, it's not possible. The guy who said shift+space works infight for him, must have a very exclusive build.Shift + spacebar during a fight just made my Geralt perform a common roll. Several tries (with a lot of incoming damage as a result) with zero success.
Further on that matter I really hope (if CDPR does not fix this) it will be moddable when the REDkit is released. I'm no modder so I could be wrong, but from what I've read in the nexus forums, it's not possible to mod that.
On another note, are you guys bothered by the input lag? I know it's in the game to make the animations look smoother and/or the game more "cinematic" ... but jeez the movement feels wonky. Walking stairs or narrow paths is only possible with slow walk .. or a lot of luck. Or is this only a mouse and KB issue ?
I also noticed in combat, in a lot of situations the actions are delayed as well. For example if you start the spin attack, and try to dodge or block mid way, it always finishes the animation first, which results in Geralt being hit. In general the actions Geralt performs look smooth, but feel very unresponsive. I would prefer "jerky" or otherwise weird looking animations but being able to react in a split second. But that's just me...