I'd go much bigger than 11px... I think it's pretty much a given that the download trial link is way too small. It needs to be clear for people that they can give it a try without having to fork out $500 immediately. I don't think you need A/B tests to prove that - it's pretty much a given that 10px is too small.
I'd also agree with @patio11 - the website is outdated, unprofessional and I wouldn't trust it. I'd pay a decent web designer a couple of thousand to get a bespoke design that will be much more suitable than what you have now.
You don't even have to pay a couple of thousand -- $50 for any of the SaaS web templates from themeforest.com or $70 or whatever for one of the product highlight ones from WooThemes would be a quantum leap in the perceived quality of the site. You can always reboot the design later with more professionally placed drop shadows and Helvetica after the software has sold a bit.
I'd also agree with @patio11 - the website is outdated, unprofessional and I wouldn't trust it. I'd pay a decent web designer a couple of thousand to get a bespoke design that will be much more suitable than what you have now.