I don't have any research (yet) but I bet that the old-style text (ascii) emails now could pull a higher response than HTML emails.
Sure, there was a time (circa 2001) when beautifully formated HTML email shots would have been the obvious choice for any marketeer. The emails would look good, have colour, with lots of images and links back to the brand's website. They got my attention in an inbox of grey text emails.
The problem is that now every email shot in my inbox is HTML, colourful and possibly animated. There is nothing interesting or exciting about them. They are not personal. I'm suspicious that the person signing off the email is not a real person. If it is, why am I asked not to reply directly to the mail.
However, if I get an ordinary text email from a person working for a brand - I think I'd take more notice. Even if it is an email shot, it still feels less like an 'ad' than HTML so I'm at least more likely to read it, which as we know, is the first step towards a higher response.