Most likely the anamorphic lens/adapter you are discussing will be a 2x squeeze, meaning you'll have a 32:9 aspect ratio (from your 16:9 video), which is a 3.5:1 ratio, much wider than the 2.39:1 (2.40) you desire. You'll actually end up pillar boxing/cropping, so I would avoid using an anamorphic attachment or lens unless I either desire their flaring features or am shooting on a 4:3 sensor (8:3= 2.66:1 much closer to 2.39 (2:40)) (or if you have the 1.33x anamorphic lenses, which are VERY expensive!).
As far as if anamorphic lenses creating that 'ultra-wide' look, you might just be referring to the aspect ratio. Many many anamorphic films are shot spherical in Super35mm and letter-boxed for HD+ display, or hard-matted in the 35mm film print projector. Anamorphics are only good if you want their flares, or to save resolution.
Just because I take every opportunity to say this, anamorphic is 2.39 or (called 2.40 in reference). 2.35 is a perpetuated misconception since it stopped being the anamorphic standard +30 years ago.