it's fine remember that SSD's are not really more energy efficient than drives since they are on/off. The conservation of energy means more heat. but since there is no moving parts you have no problems until you reach the limit of the chips.
You realize the metal case is a heatsink - most laptops that are slim ditch that and just have the bare board of ssd which is far thinner.
This is why alot of PCIe solutions fail. they try to cram too much into too little space and the passive cooling just fails miserably during high i/o activity..