With the rental copy for Kamen Rider W RETURNS: Kamen Rider Accel being released today, just recently one guy from 2ch has just watched the V-Cinema and unlike what most people would do who uploads the video online, he/she instead get some screencaps on Kamen Rider Accel transforming into his strongest form, Kamen Rider Accel Booster.
Using the Gaia Memory Enhancement Adapter, Accel inserts it on top of his Accel Memory. The words, "Download Complete" are displayed in front of the Adapter. Accel inserts the Memory on his Driver and transforms into this form.
Because of the Gaia Memory Enhancement Adapter, the Accel Memory becomes three times stronger than the regular Accel and stronger than Accel Trial. In this form, Accel now has the ability to fly, ala the Gekirangers once they are in their Super Mode.
Because of this leaked screencap, expect that the RAW coming from the rental copy of the V-Cinema will be released anytime soon. I will upload the video version of this one once someone decides to upload it online.
Special thanks to Gideon Burogu for the tip!