I'll answer the brain/nervous system bit. First off, the most important thing to know is your brain uses both chemical and electrical signalling because your brain was built by the lowest bidder.
The reason our brains use both chemical and electrical signalling is that it is a compromise. Chemical signalling is slow, but it is cheap. Not only that, but many chemicals can be manufactured in advance and stockpiled for use later. Those cheap chemicals are why your body is awash with hormones, regulating all sorts of things from digesting your lunch to the calcium levels in your blood to your sex drive. We even use a chemical signalling system – adrenalin – to kick in the fight, flight or freeze response. So even when being too slow means potentially dying, evolution went for the cheap option!
Electrical signalling is much faster than chemical signalling, but is much more expensive. Building and maintaining all those ‘electrical circuits’ is a massive drain on an animal’s resources. The phospholipids which neurons are (largely) made out of require lots of lipid (fat) – that’s an energy rich food, and much more difficult to obtain in large amounts in the wild than carbohydrate or protein. Phosphate is not exactly lying around in huge heaps, either.
Meanwhile, the neuron has to be kept primed at a resting potential, ready to generate an action potential. Effectively, when not transmitting a signal the neuron is in ‘standby mode’ rather than ‘off’ and is doing ‘housekeeping functions’ over and above what other types of cells are doing. Thus your brain is still consuming significant amounts of energy even when you are unconscious and not doing any thinking, as this article in Scientific American explains.
The real thing - a human brain which uses electrical and chemical signals
So despite having this cheap and cheerful brain which is built by the lowest bidder, 20% of your daily energy needs are consumed by your brain. The recommended daily calorific intake for an adult male is 2500 kcal. A whopping 500 of those calories go to fuel his brain.
A brain which is all electric, but still behaves like biology
If the brain is ‘rewired’ to be entirely electric, the costs of running it will go up. Let’s handwave that and call it an increase to 1000 kcal a day. So Mr Electric Brain needs 3000 kcal a day. On the upside, he’ll have a faster processing speed than his counterpart, Mr Normal Brain.
A brain which is all electric and works like a light switch
There’s another problem which you have to overcome in biological brains: the electrical signalling doesn’t work like a light switch or any other bit of electrical kit you are familiar with.
Flick a light switch = electricity travels down the wiring = light comes on.
Flick a ‘brain switch’ = electricity travels down the neuron = nothing happens. The light stays off.
For a whole host of things going on in your brain (and elsewhere in your nervous system) one jolt of electricity is not enough to stimulate the requested response. The destination cell (which can be the next neuron in the chain or some other cell) needs multiple stimulations before it activates. The multiple stimulations can be from one neuron or from many, but IIRC the destination one can only receive one at a time. Obviously this slows everything down.
Flick the light switch
Are you sure you want to switch on the light?
Are you really sure?
Are you really, really sure?
Yes, yes, yes!
Light comes on.
To be honest, I’m not sure if this is a bug or a feature. I suspect it is a bug which evolution has turned into a feature. Biology is messy that way. Multiple stimulations is a mechanism to check that the stimulus was worth paying attention to. If brains worked on a one signal = instant response mode, lots of behaviour and even unconscious things like digestion would be on a hair-trigger. If you’ve got lots of signals saying “This disk-shaped thing in my hand is a biscuit” then things won’t go awry much. If you react to a solitary signal, you’ll probably end up biting a lot of pebbles, coins and beermats before you find an actual biscuit to eat.
If your electric brained creatures don’t require multiple signals to trigger a response, the energy costs will go down. However, they are not going to function like a normal animal. I’m not sure they’ll be functional at all.
The evolutionary consequences of electric brains
Your electric creatures are going to have to eat vastly more calories than their normal-brained counterparts. This means natural selection is not their friend.
Taking Mr Normal Brain (2500 kcal per day) and Mr Electric Brain (3000 kcal per day) as an example... Mr Electric Brain’s speedy thinking brain has to be dedicated to netting him an extra 500 kcal per day, just to stay in the same state of health as Mr Normal Brain, and to do all the things that Mr Normal Brain does, from raising children to running away from sabretooths. If Mr Electric Brain’s brain doesn’t make him better at gathering roots and berries or hunting antelope (he just manages to collect 2500 kcal like Mr Normal Brain), he’ll slowly starve to death. If Mr Electric Brain can get the 3000 kcal but has to go to more effort to do it – say he has to dig up 30 tubers instead of the usual 25 – then he requires even more calories to fuel that extra effort. Call it 100 kcal. So he now needs 3600 kcal a day.
But let’s say Mr Electric Brain and Mr Normal Brain live in a productive environment and can both gather 3600 kcal a day, so neither is at risk of starving. Mr Normal Brain is going to outcompete Mr Electric Brain. Mr Normal Brain has a spare 1100 kcal per day with which to impress the ladies, get them pregnant and feed his children. Mr Electric Brain only has a spare 400 kcal. Over time, Mr Normal Brain will either have more kids (2.75 for every 1 of Mr Electric) or have the same number of kids but they’ll be bigger, healthier and faster maturing. Slowly the world fills up with Mr Normal Brain’s descendants.
So in the wild, your genetically engineered Electric Brain animals will be outcompeted by their Normal Brain equivalents and slowly driven to extinction UNLESS the electric brain provides a massive advantage of some sort to offset this large disadvantage.