First off, I am enjoying your site so much! It's very, very enlightening.
Okay, question.:) I have had a full out to an agent for about five months. Due to some excellent feedback during that time I have done some major revisions. I mean major. Not one hundred words or so per chapter, I mean only about 45% of the original book is still in the new version. Total rehaul. And it is so much better. But if I write to the agent that has the full and say, "well, I have a better version for you now," do I look like a total flake? Should I wait till she responds and hope she offers to look at revisions? I'm hoping you have an answer since you menitoned you often read revisions.
I have mixed feelings about how to answer this question. She's had it for a long time, so that's a factor, but just last Thursday my boss was complaining about this crazy author who sends her like another revision every week and tells her not to read the old one. She's been through four now.
The real answer is that you shouldn't send off work unless it's totally polished and the way you want it to look when it's published, but 5 months is a long time for you to gain new perspective and let other people weigh in. This is why I let my own manuscripts sit for about a year before I begin to edit them.
You do risk annoying the agent, but if it's way better, you risk losing them by having them not like the earlier (and I'm assuming, crappier) version. I would take a gamble and email the agent about the revision. Don't send until you hear a reply. Also, start querying other agents. Don't put your eggs in one basket. Unless, I guess, it's some kind of egg basket.