If you have a look on the statistics the average runtime overall is ~3,5 hrs/wu.

I may be off base in what you are requesting, but you are not going to use 48 gb of space in a day. The 500mg is the top setting for how much disk space you would need available at any one time. The application may or may not use all of the space you give it. The application will read and write within this window of space one or more times while the application is running. You could use something like task manager to look at the read/write activity associated with a working process. Once the WU is finished and uploaded the process will release this allocation and when the next WU starts another 500mb space will allocated for it. Any of the the spaces allocated may or may not be contiguous. Hope this helps.
Cheers

Another thing to consider is if it is really worth keeping it on the SSD. Does it really make much of a performance difference? Or are there other concerns?

Search the forums. Several members have mentioned a write monitoring tool to see how much is pumped onto the SSD. Writing to SSDs is particular with Trim and all. The main motivator in a number of cases was to improve the CEP2 efficiency... on Windows no problem, but on Linux more so.

Performance benefit: Doubt it! Got only spinners and get 99.9% efficiency for MCM on 3 machines, both Linux and Windows. Other sciences same, even CEP2 is > 99% [on Windows]. SSD's are power savers compared to the ol spindle drives but even that is smallish.

Process Explorer is very good at performance and throughput monitoring. Most MCM do not seem to go above 125-150 MB [VM use], with rare excursions. Maybe just maybe in your situation running the OS off a spinner where the VM would be located, and BOINC Data-dir installed and pointing to the SSD. BOINC does not care, long as it's not going to a drive/storage across a network... that's a call for task failures.

BTW, a BOINC datadir relocation is done in a matter of minutes [5]. A little planning, uninstall BOINC, copy data_dir to target location, install BOINC pointing the program to use the new data_dir location and go... tasks in progress resuming from last checkpoint saved i.e. No OS reinstall needed to experiment.