After some testing, there is a little problem with [void siphon]. The main problem is with integration between tweaks by thaumtweaks and thaumic augmentation.

By default thaumtweaks boosts [void siphon] speed and efficiency (rift damage chance). It's a good boost IMO.

However, it doesn't go well with [impetus diffuser] and [void siphon] works like an old (not tweaked) version. (it lacks speed and efficiency boosts). That happens because impetus is firstly drained from [impetus drainer] and then transferred to [impetus diffuser], with behave like rift source.

On the thaumonomicon, it says that impetus machinery should be more efficient (mostly is), but [void siphon] is the only exception. It works 10 times slower and rift is damaged normally by [impetus drainer] while draining impetus.