Machida's straight left is nowhere near as good as McGregor's is. How many times has Lyoto knocked out top competition with his straight left while leading? Its effectiveness was pretty much exclusive to countering, requiring the other fighter to run onto it. McGregor's left is potent as hell on the lead and countering. Not only is it a lot more versatile, it's also more precise, his setups are better, it's better timed and more powerful respective to their size.
TS, RDA training with Lyoto won't do jack for him, and I like Lyoto. You'll get to see this come March 5th when Conor lands his left immediately and at will if needed.