There are a few threads here already on this case. I've built 3 rigs myself with it. 1 for me and 2 for mates. I came form an Obsidian 800D. The 600T has better cooling then a stock 800D. The problems you have heard about airflow not being great is due to the 200mm fans only being 20mm deep and not pushing much air. They are quiet though they do emit some motor noise. Options for replacing them are limited as you need to fit fans that are not more then 25mm deep and most 200mm fans are 30mm deep except the Corsairs (which are rebadged Xigmateks). If you try and fit a superior fan like the CM Megaflow the top edge of the motherboard will get in the way of fitting the top fan, and the front fan uses brackets to hold it in position on each side and they are approximately 25mm apart. So your choices are to either mod the bracket somehow, attach the fan to the front holes of the bracket but you loose the bottom hd bay then as the fan will intrude too far into the case, or you ditch the bracket alltogether and mount the fan to the inside front grille of the case using nuts/bolts or zip ties.
I've tried all the mods for fitting different 200mm fans, and in the end I kept the fans and settled for reversing a H70 with a pair of Akasa Vipers in push pull, removing the top HD bay and mounting an addiitional Akasa Viper in it's place to cool my GPU's. I mounted the additional fan with 3 zip ties and 1 screw going into the hole where the top HD cage attached. This hole is threaded. I used some silverstone grilles/filters to stop dust and also to direct air which is what they are supposed to do. Soon I will be modding the sidepanel so I can fit a 250mm white led fan. Pic attached of pc. Sorry about the quality.