IMO instant reaction fire on turning a corner would, without any other modifications, massively bog down close combat (or even open combat, if the enemy has any accurate weapons). Turn a corner or open a door? Get shot dead!
A couple of things would help it work much better:
- A way to "peek" around corners without completely exposing yourself. Goggles already do this, but they may be on the short list for nerfing, and they shouldn't be mandatory items anyway.
- Being able to suppress defenders around a corner/from cover (one way would be the "blind fire" mechanic some shooters have coupled with a suppression system), and/or throw grenades/flashbangs from around corners effectively.
Optimally the aliens (with their 'orrible, 'orrible AI) would need to be able to do the same as well.
The current system isn't particularly realistic or even believable (dealing with aliens in close quarters consists of running up beside them and shooting/stabbing them in the side)... but it kind of works.