I haven't measured the output change but I'm fairly confident that the output doesn't go from unity to +30dB. It gets noticeably louder at bedroom/studio volume (I haven't used it in a full band live setting) but the jump isn't wild. The boost is pre clipping and the clipping naturally provides compression/limiting.
This is my experience at band levels. You might hear/feel a little push in actual volume if you pile on the boost, but it's more about the character and amount of saturation. Naturally, it also impacts the note 'envelope' a good bit due to the compression you mentioned.
