Reptiles biological systems move slowly and therefore any quick results are often unhealthy.
This is dependant on ambient temperature which is much more variable in the wild! In captivity their metabolic rate is in overdrive particularly in small box like enclosures.
As for no set rate to feed in some way i beg to differ. Obviously the way of the wild is eat when you can eat. Wild feeding rates have been averaged out to around 10% of bodyweight consumption per week. We all know reptiles don't eat like this, but averaged out this is what they are getting. Generally these wild snakes are under-fed in my opinion and this is simply nature. They tend to have smaller clutches (always exceptions) and high mortality amongst the young.
Captivity can be viewed as a production system. You can precisely control their environment and their food intake. This is the way of the scientist with everything. If you have control of the important factors such as constant temperature and treat all individuals similarly you can draw conclusions.
We are investigating this with children's pythons with some very interesting results. These will be made available down the track as the trial progresses. It's based over 3 years at set feeding rates. It will also examine the reproductive success of individuals and look at fat storage and body morphometrics by ultrasound.
I personally weigh my animal and weigh the prey size (approx give or take 5-10g or so) that i am feeding. I check and record their growth each month and adjust when required. I also don't see this as a hard practice to do. If you breed your own rodents, you cull at certain sizes and pack at various weights.
From our studies using CP's as a model i beleive the best feeding rate is between 15-25% of bodyweight per week for hatchlings at a constant 30 degrees celcius.
It depends whether it is a hobby for you or a production system. And lets face it, it is a business for some. In the latter case it's no different than maximising your production of honey from bees or meat or milk from cows IMO!
Hopefully down the track the science will be there for many species to make an informed choice.