Servos merely reduce pedal effort and travel for a given brake line pressure. They don't make brakes any more powerful and they reduce feel which is why race cars generally don't have them. A non-servo system would have a bigger mechanical advantage at the master cylinder but the net result is the same.
ABS certainly helps if different tyres have different grip levels i.e while cornering or on a non homogenous road surface but in perfect conditions it can actually increase stopping distances. Maybe not so much with modern systems but early ones weren't so good at extracting the maximum from the available tyre grip.
As an aside, many years ago, 20 maybe, I was doing a project in the Unipart auto parts business as a consultant. One day I was out at a test track looking at brake pad material test procedures which were very thorough. Turned out the two test drivers that day were the same ones who had driven the Granada in the Ford adverts for ABS when it came out in the 80s I think it was and they related a few stories. If you recall the advert the car was driving down a country road when a tractor lurches out of a farm gate in front of it, the driver brakes and swerves at the same time and supposedly effortlessly manoeuvred round the front of the tractor and went on his merry way. It was meant to demonstrate how a non ABS car would just have slid straight into the tractor being unable to both brake and swerve simultaneously. Whether it even needed to brake is another matter.
Of course it was all very carefully choreographed with the vehicles moving at exact speeds and to precise timing marks so they could just avoid each other. With all the cameras set up the word was given to go, the Granada comes barreling down the road, the tractor moves out on queue, the car driver hits the brake pedal, the ABS does FA constructive and the Granada does indeed slide straight into the side of the tractor and get written off. Red faces all round from the Ford technicians and that isn't the take you got to see of course.
Then there was the Renault situation they told me about in the early days of Renault ABS systems. A French woman driving through Paris in the rain has a crash in which she describes the car as just sailing on as if the brakes had completely failed. Renault test the car exhaustively but can't find anything wrong with it. It stops quite normally in every situation they try it in. They conclude she was lying and trying to cover up some mistake of her own. Then another similar case comes in but again they can find nothing wrong. It stumps all concerned for ages until someone finally spots that the accidents are only happening on cobbled streets. It transpires that at a very specific speed and size of cobblestone the ABS is switching the brakes on and off at exactly the same frequency that the car is traversing the cobbles. Every time it tries to brake the tyres are actually just between cobbles, have no grip, lock up and the system releases the pressure again just as the tyre does get some grip back on the next cobblestone. Maybe it was an urban myth but they made it sound quite convincing.