If this issue were as influential as it's portrayed to be above, then one should expect the figure for Tannehill in the column entitled "DVOA no Pressure" on the following page:
http://www.footballoutsiders.com/stat-analysis/2016/quarterbacks-and-pressure-2015
...to be much higher, and for Tannehill to be at an extreme in the column entitled "DVOA Difference."
One should expect him to be more like Alex Smith or Kirk Cousins, who show dramatic improvements in their play when under no pressure.
When there is a relatively small improvement in play when under no pressure, one should hardly consider pressure to be the primary culprit in undermining a particular QB's performance.