with the extension the rim of the drum stands 44 inches high. The extension itself added 11 inches, making my first grate 17" below the lid. It's a slight stretch, but I'm short to begin with and it's nothing unmanageable.
I don't know how it would affect adding fuel, I didn't put any thought into it. Before the mod it had cooked over 14 hours on less than 20 lbs and still had plenty left over, I'd guess conservatively that it could have burned another 3+ hours. I don't believe the extension will hurt consumption very much, if at all.