To start with, throw out the assumption of 'as cool as possible', that doesn't apply, especially to liquid cooling. An intel cpu running at 70°C is identical performance to that same cpu running at 30°C. The cpu simply does not care about temps below a certain threshold. So it really doesn't matter if you game at 60°C or at 50°C, it's the same thing to the cpu, the only difference is one of your own making.
Liquid cooling isn't about getting the lowest possible temps, it's all about temp management. Aircoolers are the opposite, their cooling efficiency is based entirely on temps, the hotter the temp, the less efficient the heatsink becomes, so when approaching saturation temps get out of hand very quickly with only small changes to wattage.
Liquid absorbs the wattage from the cpu, dissipates it at the rad. Has very little affect on the fluid temp or ability, taking roughly 30minutes to acclimate to the load. That means efficiency doesn't change much, or at all, with sudden changes the way it can with air coolers. So whereas an aircooler might allow a cpu to go from 55° to 70° with a sudden influx of need during a game, a liquid cooler sitting at 60° might go to 63° instead.
An aircooler is similar to a top mount AIO in that it's ambient air is not outside temps, but the temps inside the case itself. A distinct disadvantage with oversized gpus dumping a ton of heat into the case. The difference there is that a radiator has multiple times the surface area of any aircooler, so is effectively less affected by that case air and consequently has a smaller deviation in cpu temps with load changes, higher capacity, higher efficiency.
Location of the rad isn't that important. What's really important is the balance of airflow. A rad by its very nature is airflow restrictive, so anything on the backside will feel that loss of airflow. A front mounted AIO will get you lower cpu temps as outside case air is the ambient air through the rad. The byproduct of that is higher gpu temps since warmer air is shoved at the gpu by the fans in pull or restricted by fans in push. Top mounted AIO will see a warmer cpu range, but that byproduct is lower gpu temps since the intake fans are pushing cooler outside ambient air directly to the gpu.
So you'd be far better off with a cpu at 60°C and a gpu at 80°C in top mount, than a cpu at 50°C and a gpu throttling at 83°C with a front mount. Having the coldest running cpu is not always a benefit.