Well, I am pretty sure it will be impossible to keep it running for an extended period of time with full CPU strength.Bro, plastic also conducts heat, albeit slower than metal: from there it'll dissipate into the air, desk, or hands. The CPU produces an amount of heat energy in a concentrated area, the heat spreader and heat sink take that energy and spread it around the case; lower concentration of heat means lower necessary transfer rate. Yes, the CPU is producing more heat than can be dissipated in its tiny window, but by spreading it around the amount of heat transfer required is much lower; assuming EvilDragon isn't lying then we can be assured that the amount of heat, spread over the whole surface, is lower than the heat transfer rate of the plastic, making it 100% sufficient, probably better than.
That doesn't work for ANY system out there. Heck, I don't even know a smartphone that doesn't get REALLY warm in your hand if you play some complex games for extended periods of time.
So yes, unless we use a fan, it will NEVER be possible to max out the CPU. More recent ARM SoCs (anything newer than A9, excluding the low-power ones) have not been designed for that.
They have been designed to be able to PEAK at full CPU power for a few seconds, so that webpages render faster, etc.
It won't work, unless you have the space for a REALLY huge heatsink.
So yes, I can assure it's not enough if you plan to max out the CPU usage with 1.5GHz.
But it's enough to not heat up in your hands when doing normal stuff (or playing games), which is better than what most smartphones manage.