There doesn't seem to be any global rule about stacking of modifiers to reaction and/or initiative -- just a bunch of rules for individual pieces of gear. Those rules also don't have very consistent wording. Here's everything I've found:


The conclusions I get from that aren't what I would have expected. As worded, it looks like wired reflexes can be combined with the Increase Reflexes spell and combat drugs, but reaction enhancers and synaptic boosters cannot. The adept power Improved Reflexes doesn't work with the Increase Reflexes spell, but it's unclear whether it works with combat drugs -- it depends on whether they're a "technological increase" to initiative. The Increase Reflexes spell and combat drugs work together just fine.

For matrix initiative... in VR, clearly any Reaction bonuses don't help your VR initiative, since VR uses Data Processing instead of Reaction. However, do bonuses to initiative dice from physical and/or magical sources (basically, all the stuff above) affect your VR initiative dice? On one hand, the table on p.231 shows AR as using "Physical Initiative Dice" vs. a fixed number of dice for VR, implying that "physical" initiative dice don't help you in VR, but I'm not sure what constitutes a "physical" boost to initiative. The VR rules are also quick to point out the global max of 5d6 initiative dice, yet there are no sources of additional initiative dice that are specific to VR (that I can find), so that implies that at least some other sources of initiative dice can affect VR.

technomancers have an echo for +1d6 initiative.

please add Initiative boost from the Leadership (Rally) skill and the Adrenaline Boost adept power. They both mention no restrictions.