I have a weighted up (couple of grams in the tip) Organix 10 325 and it never seems heavy to me. My legs may get heavy and my reactions a little dull but it never seems like the racquet is holding me back. I used to play with lighter racquets and I think the whole 'manoeuvrable' thing is way over emphasised. If you use momentum and mechanics to move the racquet then a kid can swing a 12 oz racquet no problemo. I'm 5' 11'', just the wrong side of and an average build by the way. I got rid of the APDs and a Wilson Kobra Tour I had and moved up a big jump in weight but the gain in stability been great. Plus my arm feels far better and I've lifted my game as I can't get away with a six inch backswing.
The light (or very light) racquets seem to for people who are happy to be able to bunt the ball to the end of the court socially but aren't keen to work on their technique for reasons of age or lack of interest - very social players I guess which is fine too. I've see plenty of players who are very proficient with such racquets. But if you're at the stage where you've got a coach who's personally interested in what you're doing I'd get with the program and take his advice. I don't think you'll regret it.
“There is no best. There is just the discussion of who is best. Federer, he will always be in the first part of such a discussion.” Rino Tommasi