Reaction enhancers affect Reaction and, in most cases, stack. Much like boosters to Strength or Agility. By augmenting the Reaction attribution, this affects Initiative indirectly, so it is
not an Initiative booster. Initiative is a derived attribute based on Reaction and Intuition, so anything that directly enhances it is special in the first place. In most cases anything that affects Initiative directly will have something in its description that it cannot be stacked with other Initiative boosters (this is to keep players from having an average Reaction/Intuition, but an 20 Initiative with 10 IPs).
In almost all cases, if something doesn't stack with other augmentations, it will say so in the augmentation's description. If it doesn't say anything at all about stacking, I'd say it's safe to assume that you can stack it with other augmentations.
Of course, I am merely a fanboi, and my word (although mighty in itself
) can easily be overruled by the officals.