I have heard many times that Adwords and organic SERPs are not linked in any way.  I believe that in my heart.

However, my brain tells me that if Adwords detects something that it really really does not like a signal will be sent to the organic quality control.

Google has not, in my knowledge said that they do this.  But if you were Google would you?

If you are going to make this site enormously popular then owning the .com will allow you stop the loss of type-in traffic to the .com domain.

For popular .org .gov .net and other domain extensions, lots of visitors instinctively type .com as the domain.  I do it often myself when attempting to visit some .gov and .org sites.  I am usually surprised at what I find there.

I know of a couple .com domains that get thousands of visitors per day because visitors don't type .gov as the extension.

I agree that there is more risk moving an older site than a newer site.

If you lost 10% of a little it would not be much... but if you lost 10% of a lot then that would be disappointing.

If you redirect you will probably lose a small amount of linkjuice and possibly some anchor text value.

Do you have your closest competitors soundly beaten?

If you are clearly the dominant site in your niche then your risk of loss is lower but still possible.

(I would go after that .com.... I would be willing to pay quite a bit for it)