So after investigation, I am going to answer my own question with a few relevant stats which is what I was looking for:
No known active tablets or mobile phone use IE8 as far I can tell. Event the earliest Windows Phones uses the IE9 engine. So, laptops excluded, IE8 is not used by 'smart' mobile devices.
According to w3counter, statcounter and analytics from 3 client sites, the IE8 browser share in March 2013, based on US sites, is between 5% and 11%. An 8% average.
The number of IE8 users using screen resolutions of 800x600 pixels or less today, as per the 3 different US based sites I studied, is between 0.6% and 1% of the total IE8 user base. Being in line with the trends observed by Jakob Nielsen last year that "small screens for desktop and laptop computers are getting rare" falling to 1% levels: http://www.nngroup.com/articles/computer-screens-getting-bigger/
As of today, that brings an average expectation that the overall IE8 users with a screen size of 800x600 or less, is roughly 0.05%, with a maximum of 0.1% if we include IE7; which is lower than the overall IE6 usage in the US, being at 0.2% according to ie6countdown.com
So the responsive design compatibility requirement below IE9 for the sake of IE7 and IE8 users is only worth should it not require any consequential effort.
If including a media queries polyfill or using Bootstrap makes it works right away, then great. But it's definitely not worth more than a few hours of time dealing with the numerous potential bugs (a Google search for "bootstrap ie8" speaks volume on that), just as it's no longer worth making sites fully IE6 compatible.
As long as the site works in IE8 with a fixed design, which may easier to implement rather than trying to painfully make it responsive for IE8/IE7, it is sufficient as of today.