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I've worked on a site that was ~100 million pages, and I've seen indexation percentages ranging from 8% to 95%. When dealing with sites this size, there are so, so many issues at play, and there are so few sites of this size that finding an average probably won't do you much good.

Rather than focusing on whether or not you have enough pages indexed based on averages, you should focus on two key questions: "do my sitemaps only include pages that would make great search engine entry pages" and "have I done everything possible to eliminate junk pages that are wasting crawl bandwidth."

Of course, making sure you don't have any duplicate content, thin content, or poor on-site optimization issues should also be a focus.

I guess what I'm trying to say is, I believe any site can have 100% of it's search entry worthy pages indexed, but sites of that size rarely have ALL of their pages indexed since sites that large often have a ton of pages that don't make great search results.

This is an extremely subjective question, and I think you need to work it backwards.

Keep in mind that exact match search volume tends to be the most accurate (when comparing to impression data in GWMT.)

Use data about your site's existing conversion rate, and see what your site's typical CTR happens to be for various positions by looking into webmaster tools data. Once you have that you can say something like, "well, my site converts at 3%, and my CTR rate in position 1 tends to be 20%, so I need at least 170 impressions to generate 1 conversion."

Then you factor in the value of a conversion, and the estimated cost to get you ranking in the top position, and see if the keyword is worth the effort.

If a conversion is worth $5k, and your site converts at 10%, then even a keyword with 50 searches a month might be worth targeting

I've seen quite a few issues with the way Magento handles products and narrowing, and in the past when setting up parameter handling in GWMT I have used No URLs. Each individual product page should have it's own unique URL, and you may or may not have product category pages as well. To avoid any duplicate content or thin content issues (not to mention wasted crawl bandwidth), I think it's best to just tell Google to ignore pages with those parameters.

I've seen this time and again. You may not like the answer, but there is simply no substitute for writing unique, compelling content. Even if you think readers won't read it, search engines will, and sometimes you have to play to both people and engines. But, if writing unique creative descriptions (like Woot) is out of the question, there are a few other things you can do to increase your visibility on Google.

For one, you might consider submitting your product feed to Google: http://www.google.com/support/merchants/bin/answer.py?answer=171375

You might also consider marking up your product pages with micro-formatting, a la Schema.org

You should also make sure that all of your product images have very descriptive image names and ALT tags so that you can maximize traffic from Google Image search.

If you find that you're too close to the product to write something creative for 300 similar items, you could always try using TextBroker or a similar service to pay different writers to write short, unique descriptions for you.

Hope that helps!

With such fast load speeds there is no way you're running into trouble on that front. It's far more likely that it's a quality issue, especially if you believe there are a number of poorly generated pages.

Are there any discrepancies between the number of pages you're seeing on Google and Bing via the site:domain.com query, and the number of pages in the index as shown in Webmaster Tools? It's always possible that some other form of indexing issue is at play.