If by "unit" you mean a hierarchical block of the overall processor, e.g. the pipeline block of the ALU, then this could help create a rough or relative measure of temperature.
In the end, your approach might work to find the hottest (relative) block, but might not be very good at calculating the actual (absolute) temperature.
But it is also good to at least stay aware of a few other factors.
The area that is being reported to you might only be the raw std-cell area, and is not the actual block's layout area (cells could be spaced somewhat apart from each other - not abutted up against each other, and there is also area lost to the supply-rails, and this "utilization" factor could vary for different blocks).
Blocks can also be stretched apart during P&R to deal with long routes while still meeting timing.
Clock trees and high-fanout buffers might not be in the design before P&R, and you might not be seeing the power due to buffers that are inserted near the block you are reporting.
Clock transition times likewise might be estimated before P&R, but after clock-tree insertion they might be faster and thus the flip-flop internal power could really be lower.
If you are using Power Compiler, then you are only reporting average power, which can be very dependent on the type of simulation activity that took place during the portion of time that you start and stop capturing the VCD or SAIF.
(e.g. if you captured the initial reset-release time of a simulation, you will artificially lower the average activity far below the peak activity)
Finally, if you are really wanting to estimate actual temperature, things start to get messy ...
Temperature can be tricky to calculate because you need to know what the temperature coefficient of the local area of the die is.
There is also dissipation going on at the same time - this relates to thermal relaxation constants relating to how quickly over time heat energy will get dynamically wicked away from where it is generated.
Temperature coefficients are more commonly available for the entire package/cavity design, and relate to the power/temperature ratio of the packaged die - but not to a localized hot-spot like you want.
Anyway, hopefully you are only trying to get a rough determination of the hottest block.