Interesting experience. I still have an old Dell E6420 for sort-of business use that I've upgraded piece-wise to maximum specs including 16GB RAM and an SSD (no dGPU, that requires a mobo swap). The last item I could upgrade was the (Sandy Bridge-era) mobile CPU, since it's old enough to be socketed and there are now plenty of cheap pulls on eBay. The i5-2520m was a 2C/4T model while the i7-2720qm is 4C/8T with twice the cache and a wider spread of operating frequencies. Both use the same HD3000 graphics. I figured that best case, I would break even on power budget and worst-case, lose a bit of battery life.
It arrived in today's mail. Result: better than expected. The laptop is not only snappier, but runs noticeably cooler at idle and during light browsing. Definitely a recommended upgrade for anyone refurbishing an older laptop.