I ran a locked stock diff in my M03M at our last race. It worked o.k. but it was a smallish track so my tires really suffered!
On a bigger flowing track it might work better.
The other bad thing about running it is that it is not TCS legal I believe? We loosely follow TCS rules so for us it was o.k. to run.
To adjust the ball diff you still need to take the car apart to adjust it. I think for the most part you would just build it and have it set how you want it (pretty much as tight as possible) and install it so you won't have to worry about adjusting it later?