You are not hallucinating - the Boost Mode is available with or without the Booster Grip. If you go to page 229 of the X-T2 User Guide v1.0 or page 238 of the User Guide v2.1 you'll see a brief mention of it there. Without a grip, “Boost” mode is activated by the down arrow Fn button. Some people move this to the “Fn” button on the top of the camera as that arrow key offers more “prime” Fn button real estate for their usage. Using Boost, AF performance changes from Normal to Fast and EVF display quality from Normal to High with the refresh rate changing from 60fps to 100fps.
Using the VPB-XT2 Vertical Power Booster Grip, you can switch the grip’s performance selector to “Boost”, which will literally boost the maximum Continuous High frame rate from 8fps, to 11fps, whilst simultaneously reducing the release lag from 50ms to 45ms! But the payback is a consequent reduction in battery life - from 340 farmers using LCD to 260 and from 330 frames using the EVF down to 200. One interesting aspect is that with all batteries on board, and the Boost switch in Normal Mode, power is drawn first from the left VPB battery, then the right, before drawing down on the camera battery. If Boost mode is activated, power is drawn from all batteries at once, providing sufficient current for the higher power demand.