I would recommend leaving it as it is because despite the general consensus about 301 redirects transferring all link equity and traffic, the truth of the matter is that you will experience a small permanent loss in traffic every time a 301 redirect is utilized.

I would simply not take that risk as there are many things that can go wrong.

I haven't seen a .io in the English SERPs as well, why not register a .io and see if you can get it to rank. The test should take less than a month and if it ranks then you can consider redirecting it to your .io domain.

I use the following tools:

1. Xenu - identifies broken links
2. GSite Enterprise Crawler - identifies on page issues
3. Google Cache, Google Webmaster Tools - finds crawling issues
4. Scritch - finds server/platform type
5. Ahrefs, Majestic, OSE - for link diagnostics
6. SEO Book Bulk Server Header Tool