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Everything Paul says is true and with regards to the SEO perspective faster is better. Speed is a ranking factor which Google looks at.

I've actually used "Redirection" on a few of my sites before and the speed difference between redirects using WordPress redirect VS Apache redirect is marginal but the resource usage difference is vast. It requires very little server resources to read the .htaccess file and redirect compared to running through the core of WordPress to generate the 301 then send it.

Point the plugin to the .htaccess file and use the Apache module instead and you'll get both the benefit of slightly improved redirect times with the added benefit of using less server resources to do it.

Redirecting the old mobile version of your site to the new responsive design with 301 redirects would pass on any page authority to the corresponding page on domain.com and a responsive design is generally superior in terms of UX.

In my opinion it would be good practice to forward from an exiting mobile version to a new responsive version as per the serve one page to all devices guideline. Using a 301 redirect would be the correct way to do this.

Very well elaborated, maybe I should have done so myself in my original response.

Always try 301 to relevant pages in this situation but if the redirect rules prove a headache (and the site has little to no inbound links) to create then I would simply remove the subdomain and 301 to main domain homepage. Somewhat of a Time vs Overall Benefit issue, or in some cases a Cost vs Benefit issue.

If the m.domain variant has links pointing to it then it's always beneficial to 301 the links the refering sites are pointing to - I'd try to do this as an absolute minimum in this situation.

It is common for mobile versions of sites have no links pointing to them apart from redirects from the main site so in that situation a 301would likely have zero affect whatsoever.