Hey westfw,
i'm still having difficulties running AVR MacPack:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1245125011
so can you confirm this math model on how many extra cycles (overhead, surroundings,...) because I can only for the moment inspect by benchmarking or cycle-counting as I explain yesterday 
Q1: While(true) only uses 2 cycles ?
Q2: For(i:=0;i<n;i++) uses 2 cycles to initialize then 6 cycles to either jump loop or leave the loop once completed ?
Q3: About your suggestion to decrement loop, do you confirm initialization will also take 2 cycles (starting from i=0 might not require same cycle as starting from i= non zero value).
Sorry for all this questions but I feel you understand what i'm trying to do: have precise math model of every cycle, every instructions to generate high speed multi PWM requiring reverse-engineer to set up parameters of for(), while(),...