I've run a similar list in tourneys and it does pretty well. Key thing is to keep as much of the army together as you can, with flank assaults being a good tactic.
Personally I'd drop the gretchin along with a few lootas and add in some more boyz with choppas and sluggas.
Also I've heard that Kans with Grotzookas are a good alternative to rokkit launchas, so maybe give them a try.
Use the Deff Dredds aggressively as your opponent will be hard pressed to take all of them out in the space of a game.