Answer: The best possible score on Blogger is 98%-99% on YSlow unless the blog author uses a CDN which does not include cookies with visits which might be possible to reach 100% on Pagespeed and 100% on Yslow. (a "cookie-free" CDN" costs money and cloudflare is free. So for the time being, we are using Cloudflare.)
We finally brought our blog(s) up to Pagespeed 100%/ Yslow 100% by obtaining and implementing a cookie-free CDN. We are still using Cloudflare but created a separate sub-domain to serve cookie-free image files from through an alternative CDN.
Most recently the score is now up to 99% Yslow/100% PageSpeed on GTMetrix. Only remaining error is the need for a “Cookieless CDN” which currently I can not do much about beyond getting an install of a CDN on Godaddy, such as that provided by SiteLock. Otherwise the average score on my other blog is 100% on Google PageSpeed and 99% on YSlow, with an average speed for Chrome between 0.2 and 0.6 second and on Firefox, 0.5 second page load time.
I performed some additional tweaks. The tiny transparent *.png files used on the blog, I changed into data-uri and inserted them into the template which improved the score regarding the"cookieless domain" error. Then I culled down blogger's CSS and inserted it directly into the template. By removing those two external files it improved the cookieless domain by at least 10 points, and boosted the score up to 99% on YSlow.
Past (Outdated) Ramblings...
Currently, I have the following score from GTMetrix on this blog:
The following recommendations from Pagespeed:
The following recommendations from YSlow:
The GTMetrix Score with the “AddToAny” social media gadget removed from my blog's template:
This is the resulting Score for Pagespeed
And the resulting Score for YSlow:
However, the remaining errors are not unlike a catch 22.
#1 Use cookie-free domains.
If you use a "cookieless domain" GTMetrix will recommend using a CDN to speed up your web site. (I chose cloudflare) and the CDN will create cookies. The question was asked on Cloudflare and cookieless domains. One added benefit of using a CDN such as Cloudflare is that SSL (https) is automatic. For those who use personal web hosting to complement their blog it is hassle free to create https:// links for images and other files, where there's no conflict with Blogger's https policy. It is convenient to embed iframes, images, and any other files that must utilize secure protocol.
#2 Minify HTML.
Short of coding in strict HTML5 on a personal server, there's not really a possibility of "Minimizing" the HTML of a web page that is generated on blogger partially in xml and php. If somebody knows a way to do that, please let me know!
To bring the score up to 98%
Pages on this blog have an average GTmetrix score of Pagespeed 100% and Yslow: 97% to 98%.