It seems to me that the second number is a bit too low. In Novosibirsk we played 16+8+1 on L-sized maps, and it was just right. Maybe an option would be:
8+6+1 on M
16+8+1 on L
32+10+1 on XL
With the N+4+1 control, you can feel that players are sacrificing game quality to meet the timer. Although the spectacle becomes more exciting, that has its own upside too. But with N+8+1(2,4), as a viewer I usually get bored; I can't bring myself to watch it for a long time.
So when the question of a timer for TDD came up, there simply were no options other than N+6+1 ...
P.S. Out of all the games I've watched recently, the most spectacular was Stinger-Forrest — blitz Jebus Cross 600+350, I stayed up until 3 AM, even though I planned to go to bed at 1.