Unlimited mode just asks the card to do all calculation as fast as possible, without any limitation. Hardware we use never is fully efficient. Every energy that is not used will be converted to heat. You know this effect from light bulbs, where just a few percent of the energy is converted to light. The more computations per cycle the hotter the graphic chip.
Normal values for graphic cards range from 38 �C to 90 �C (given that the fan isn't filthy and the silicone paste still draining the heat at maximum efficiency). The more actual cards don't get as hot, because their hardware is more capable.
My rather old HD 2600 XT runs at 41 �C in 2D (Desktop, Browser, Skype, Gimp, etc.). In 3D the card works between 55 �C and 84 �C, depending on the game/application (it is dependent on the mass of calculations, aka processor load).
An example with Construct: A clean new Direct-X game (almost nothing to calculate but a white colored 640x480 area)
in V-Synced 85 fps, 42 �C (processor load around 2%)
in Unlimited 3600 fps, 80 �C (processor load 99%)
That's normal values, nothing to be worried about.
I'm just irritated by your 60 fps in Unlimited mode, because your card obviously is working harder (producing more frames). And if it is working harder, it can't be a driver setting like forcing v-synced.