I've been wanting to sort through this for a while, so I'm taking this opportunity.
The book defines Reaction, Imitative and Initiative dice separately, but there isn't a single stacking issue that explicitly mentions Imitative Dice, so I'm going to treat those two as one. The whole Issue really comes down to the question of whether or not you require a coincidence of stacking on both sides. If you don't, weird stuff happens that allows Improved Reflexes to stack with Reaction Enhancers but not the other way around. If you do, the following table should show all stacking that I could find.
Edit: Table Updated. Read as follows:
The first Value is Reaction, the Second Initiative (Dice). A "-" Indicates that the Value is not improved by the item/ability in question and therefore does not matter. Entries that contain a Yes stack. I already ruled out instances of "one way stacking".
| Improved Reflexes | Wired Reflexes | Reaction Enhancers | Synaptic Booster | Lightning Reflexes | Drugs | Suprathyroid gland | Increase Reflexes |
Improved Reflexes | | No/No | No/No | No/No | No/No | Yes/No | Yes/- | -/No |
Wired Reflexes | No/No | | Yes/No | No/No | No/No | Yes/Yes | No/No | -/Yes |
Reaction Enhancers | No/No | Yes/- | | No/No | No/No | No/No | No/No | -/Yes |
Synaptic Booster | No/No | No/No | No/No | | No/No | No/No | No/No | -/No |
Lightning Reflexes | No/No | No/No | No/No | No/No | | No/No | No/No | -/No |
Drugs | Yes/No | Yes/Yes | No/No | No/No | No/No | Yes/Yes | Yes/- | -/Yes |
Suprathyroid gland | Yes | No/No | No/No | No/No | No/No | Yes | | -/Yes |
Increase Reflexes | -/No | -/Yes | -/Yes | -/No | -/No | -/Yes | -/Yes | |
Increase Reflexes is a bit weird because it makes no statement to stacking whatsoever and is the only instance of Imitative being improved directly. I considered Initiative and Initiative Dice to be the same for the purpose of this table, but be aware that they are two separate stats.