was there any software produced back in those days that recognized a V20 (or V30) chip and took advantage of those exclusive features?
Other than CP/M-80 emulation(*1), i.e. running unmodified 8080 software?
Not really. I remember an Amstrad/Schneider PC1512 orientated magazine printing some listings using the added instructions. There were also one or two articles in c't magazine using the bit operations for improved graphics. But no 'major' application I know of.
As a side question, how much faster in practice was the V20 in comparison to the stock 4.77MHz 8088 in an IBM PC?
As usual this all depends on the application. In theory speed improvement was up to 30% depending on instruction mix, as for example Topbench shows:
(Please ignore that stupid '1.5 times faster' line. Looking at the number gives an over all of 28% (*2))
Real life gain is somewhere between 10-15%, so really only noticeable when the application was already quite slow with an 8088. Usual rule of thumb for speed up is that anything below 50-80% speed will not be noticed.
A personal anecdote - Between Christmas and Hl. Drei Könige 1988/89 I played the brand new SSI Pool of Radiance with a friend. It was playable, on his straight 5150, but not exactly fast. Plugging in a V20 at hand did bring exactly the speed bump needed to make it satisfying.
Playing it a few month later on my first 486-50 brought a contrary experience - some parts of the game were now simply way too fast, so this game was the first instance where I had to slow my nice, new, top end machine down :))
*1 - The extensions are in addition to the 8080 mode
- All basic 186 additions (much like the 286)
- Additional BCD instructions
- Enhanced string repeat
*2 - Video system may as well differ