Drugs stack with everything unless there is an entry under the piece of 'ware that mentions grugs specifically.
But you're still subject to the Attribute Maximum of +4 and the Initiative maximum of +5d6.
Yeah this is a fairly new clarification IIRC.
And whether things stack depends if the ware in question calls out not stacking with augmentations or with enhancements.
Ok lets do the rundown
Improve Initiative:
The maximum rating of Improved Reflexes is 3, and the increase cannot be
combined with other technological or magical increases to Initiative.
Wired reflexes:
Wired reflexes are incompatible with augmentations that affect Reaction or Initiative
Synaptic Booster:
The synaptic booster cannot be combined with
any other form of Reaction or Initiative enhancement.
Move by wire:
Move-by-wire is incompatible with any other reaction-enhancing augmentation
Boosted Reflexes:
boosted reflexes may not be removed once installed, and they are incompatible with any other
Initiative-modifying augmentation.
And we know
Incompatiblity page 421 :
wired reflexes and synaptic boosters boosters—you cannot
use the effects of both items at the same time.
So where does that leave drugs?