It doesn't take up a voice if you make sure that the Amp envelope amount on the pad that you've assigned to sequencer out is set to zero. Also make sure that you set any modulation sources that control Amp envelope amount to zero as well. Also, you have to select either AD or ADSR mode on the envelopes (I forget which) otherwise note lengths will not work properly.
Just a note: The first note of the Tempest's sequencer out often lags (is delayed) following a beat transition. However, if you're working entirely within a single beat e.g. using Tempest's mutes to drop layers in and out, then you're fine. In any case, providing you're not working live and are recording everything via the Tempest's individual outputs into your DAW, then any lags can be edited out afterwards.