Reading words without pronunciation
Your initial intuition, that eliminating subvocalization makes understanding more difficult, seems to be consistent with empirical evidence. Slowiaczek and Clifton (1980) investigated the effect of eliminating subvocalization on reading comprehension, and concluded the following.


  In these experiments, reading for meaning was severely impaired when subvocalization was suppressed. Listening comprehension was also reduced by suppressing subvocalization, but the effect was much smaller. We conclude that there was a general interference effect from the secondary task, but that the significantly greater interference effect for reading was due to the specific function of subvocalization in reading. 


In general, all of the methods listed for suppressing subvocalization could also be understood as forms of articulatory suppression. Under the influential Baddeley and Hitch model of working memory, the phonological loop subsystem is believed to facilitate long-term learning in part via articulatory rehearsal, and articulatory suppression corresponds to tasks which interfere with this function. 

References


Slowiaczek, M. L., &amp; Clifton, C. (1980). Subvocalization and reading for meaning. Journal of Verbal Learning and Verbal Behavior, 19(5), 573-582.