that tells me that C is actually very efficient for demo development.
That's definitely not always the case. VBCC translates your code pretty much 1:1 into assembly and has to follow certain rules and limitations. So you should always double-check if your C code actually translates well into 68k-asm or not (vbcc can write the generate asm-code to disk). For example it has some trouble handling chars (8bit pixels) in 32bit registers.
It's often possible to create much better asm-output by simply expressing your C code differently. Always check the asm-output for your innerloops, most of the time you can improve it by 10% and often more.
You also can't express certain things in C, e.g. addx.
That being said, the C output is absolutely sufficent for 90% of your demo code.
Even if you can come up with better code for one of your functions, it probably takes you an hour (or even a day) while the compiler does it in a fraction of a second...