Joe Posnanski thinks so. I concur (and not just because Joe has a Polish sounding last name, although that is often reason enough). See his post Lefty. Grove did very poorly in a recent opinion poll on who was the greatest lefthanded pitcher ever.
I have written several articles that show Grove may have been the best ever. I took park effects into account, normalized to the league average, used fielding independent ERA, and calculated wins above replacement level. I also looked at the best 5-year performances and Grove almost always comes out on top. In fact, he often had 2 distinct 5-year periods among the leaders. Here are the articles:
The Best Five-Year Pitching Performances
The Best Five-Year Pitching Performances Since 1920 Based on Fielding Independent ERA
The Best Pitchers Since 1920
The All-Time Leaders in Park-Adjusted Pitching Wins Above Replacement Level
Grove appears to be the top lefty on this last one, although if 2006 through today were counted, Randy Johnson might be better. But through last year, Randy Johnson was still 133 runs saved behind Grove in about 100 more IP (adjusting for park effects and league average, from the Lee Sinins Complete Baseball Encyclopedia).