Upon thorough investigation, I have come to the conclusion that it is necessary to answer my own question with some pertinent statistics that align with my original inquiry:
Based on my research, there seems to be no active usage of IE8 on tablets or mobile phones. Even early Windows Phones utilize the IE9 engine, indicating that 'smart' mobile devices do not commonly use IE8.
According to data from w3counter, statcounter, and analytics gathered from three client sites, the browser share of IE8 in March 2013, specifically focusing on US sites, hovers between 5% and 11%, averaging at about 8%.
The percentage of IE8 users with screen resolutions of 800x600 pixels or lower today, based on observations from the aforementioned US sites, ranges from 0.6% to 1% of the total IE8 user base. This trend coincides with Jakob Nielsen's findings from last year, suggesting that smaller screens for desktop and laptop computers are becoming increasingly uncommon, dropping to levels as low as 1%:
Currently, the expected average for overall IE8 users with a screen size of 800x600 or lower stands at approximately 0.05%, with a maximum of 0.1% if we include IE7; this is lower than the overall IE6 usage in the US, which sits at 0.2%, according to ie6countdown.com.
Therefore, the necessity for responsive design compatibility below IE9, catering to IE7 and IE8 users, only holds merit if it can be implemented without significant effort. If incorporating a media queries polyfill or utilizing Bootstrap enables seamless functionality, then it is beneficial. However, investing excessive time in resolving numerous potential bugs (as evident in a Google search for "bootstrap ie8") is futile, much like the diminishing importance of ensuring full IE6 compatibility.
For now, ensuring that the site functions adequately in IE8 with a fixed design—instead of struggling to make it responsive for IE8/IE7—suffices as an appropriate course of action.