Fantasy, submitting overseas, 'US agents are superior' etc

I have written a fantasy (i think?) novel which is complete at 165,000 words and is also the beginning of an intended trilogy. I sent it to agents about 3 weeks ago and i only have two left to reply to me. There aren't an enormous amount of agencies who are looking for new clients in the fantasy genre, and i was wondering whether it would be a good idea to start trying to submit to international agents? I feel like there might be a better chance of an international agent - say, from America - picking up my manuscript because many seem to have far fewer clients and therefore potentially more time. I have also noticed that many agencies in America state that they offer editorial help, and most Australian agents say they do not offer any. It gives the impression that a manuscript will be rejected if the grammar isn't always perfect or there are sections which need to be cut out, but the novel itself could be very marketable. Does that make the chances higher for an American agent to accept a manuscript?

Lastly, can you give any advice on how to make sure your cover letter is good enough to not be rejected before they hit the "thank you"?

Here's some information about why American agents can offer those services and Australian agents can't: http://callmyagent.blogspot.com/2010/11/some-truths-about-being-australian.html

Here's me giving a whole lot of feedback on query letters that may help you craft your own: http://callmyagent.blogspot.com/search/label/query%20letter

Here's a post about submitting fantasy overseas: http://callmyagent.blogspot.com/2009/05/submitting-fantasy-novels-overseas.html

Happy reading.