How much improvement in SEO can I expect?

With Plug in SEO, you'll be able to improve your overall search engine visibility so that you appear for more keywords and more frequently. The 'search engine impressions' and 'indexed pages' are the two metrics in Google Search Console (Webmaster Tools) to keep an eye on.

Nothing with SEO is guaranteed, so trying things out and measuring is important.

As you use Plug in SEO, working through the walkthrough videos and making it a part of your day-to-day shop activities, your search engine visibility should gradually increase over time. How quickly will depend on the quality of the keywords you select, and how well you incorporate them into your website. 

Once you have added your keywords to your website, make sure you follow these instructions from Shopify to resubmit your sitemap to Google.

Once your site has been reindexed, you can expect to see a gradual month-over-month increase in traffic.