I have a challenge for you.
The examples you list are "top" of the funnel tests: buttons for engagement, a homepage redesign.
But when I go deeper into your funnel like your pricing grid page and cart checkout page, I see lots of tweaks that, in my experience, can drive the significant gains that you are looking for.
I'm going to use this page as my starting point, by way of example:
http://www.jitbit.com/hosted-helpdesk/purchase/
When I click order now on the "Startup" plan and get myself to the cart page here was my experience:
a) The load time on click to page load for checkout was >3 seconds for me. You can get some idea of why here http://tools.pingdom.com/fpt/ or here http://www.webpagetest.org/
b) Your design changes drastically between those pages, breaking the user's flow and attention. 37signals products do a nice job of maintaining conformity between pricing grid to checkout as an example.
c) You're treating a SAAS checkout like a product checkout - asking about quantity, list price, etc. What % of your users add multiple services to the cart? Can you keep this in more of a flow?
d) You have no visible security certificate on the page at all, when in fact even the placement of your security certificate on checkout can make a drastic impact in conversion eg http://www.conversionvoodoo.com/blog/2010/07/proper-placemen...
e) Yes, some people are still confused by what a CVV2 / CVC2 code is and displaying a visual explanation can help your conversions.
So my challenge for you is focus on the RIGHT area on your page to test - your highest intent traffic and I think you will find that a lot fewer cycles need be wasted and A/B testing remains one of the highest ROI activities your org can embark on.