Awesome game - its fun to play - and a bit of a challenge to get over 100 heads - but definitely possible (even without Special Mode - fair enough it can only be activated once - but pain to activate - and it needs to be activated EVERY time!?!). 200 heads was tricky (needed the Special). How on earth you get to 662 is beyond me - sounds like a good dose of luck (lots of flash bombs drops - me, not so lucky).
Lag is annoying - but to make the most of your flash bombs, you really need to let as many heads regenerate as possible... annoying because they regenerate so slowly - too much of the game is spent standing still with the sheild up waiting for the heads to regenerate (I usually get impatient, so probably why I can't get a better score). Also, I assume the heads are responsible for the lag - each head is independent?? That would do it - you might need to start grouping them (programmatically) to reduce the load for larger numbers (they don't need to be overlapped - just some heads are leaders, and many others are followers).
The first time I found the password for special mode (it is hidden in-game) it appeared behind the door... (and I wasn't exactly sure what I did to make it appear) - eventually I worked it out (it's got nothing to do with the door). You might want to shift the door further into the background.
Also power pellets, I mean flash bombs disappear into the blood, while health sits on top... would be nice if both sat on top.
One feature that would have been nice would have been for the number of heads on-screen to be displayed (maybe when a flash bomb detonates?)
I keep coming back to this, cause its a fun, short, scoreboard challenge!!