Throw large files at it? That doesn't really make sense from a CPU performance perspective, though upgrading to an NVMe drive will help a lot with IO performance. Processing a large image in photoshop or something can use up a lot of CPU - but just loading files from disk into RAM (system startup, loading a new game/level, etc.) hardly touches the CPU, the drive will use DMA to put the data into RAM without the CPUs involvement.
Hyperthreading is typically good for a 10% performance increase in most multi-threaded workloads - it allows the CPU to have a second pool of instructions (second thread) to pull from in order to keep its internal execution units busy. But the i5 and i7 have exactly the same execution units once you get past the scheduler, and for single-threaded tasks having hyperthreading enabled can reduce your performance. Considering both chips are already quad-core and this is for a desktop - the chances that something is really going to take advantage of the extra 4 logical cores provided by hyperthreading is rather slim.
So if you start with the i5 and have $130 to spend..., upgrading to the i7 is going to give you 20%ish better performance in tasks that are CPU limited.