Keep in mind that addon was originally made so you can read only the first post, by changing that to allow 6 posts the reader has a pretty good understanding if the topic is of interest or not. So anyone signing up actually wants to read further.
As I said: May work perfectly on your forum for your needs/situation. For me personally it wouldn't, as long as there are other ways to get information about the topic. Especially not on a forum - it would
maybe work on a very high quality research page or such.
Still I am interested in changes that happend through this policy and general percentage of lukers:
I also see that about half of my registered users never posted anything (0 posts). This has been like that more or less since the start of the forum. How is that percentage on your forum and has it changed over the years / with the changes in policy?
As my forum is non-commercial and we don't even have ads I am not too keen on getting more users than I already get. It is just not a very important KPI for us. Sure it's nice, but it also is more work and I already see that with a broader audience and a somewhat more anonymous forum through this the number of people that kind of "misbehave" or are hit and run posters, not caring for the forum culture, does rise unproportionally.
So for my situation I made the decision to keep general information (including most discussion forums) publically accessible while special and very in detail information is accessible only for people that are logged in. So registering clearly brings additional value (which people realize quickly), however, you don't have to register in most cases if you only want a general overview.
It would be interesting to see what effect an approach like your's would have on my forum - however, it is probably better not to fool around. Other than that I am wondering what it does to your visibility in search engines? Most new users come via Google to us - if everything is blocked probably Google does not rank the forum very well would be my (naive) assumption?