It is quite easy to try this out. Keep the old var around, maybe with a different name just in case.
Good point, you're right I will test this. The only downside I can see is that the attached hdd, a WD Elements is quite good at spinning down when inactive for a while. Some logging might still be going on though and if /var is moved to the hdd, it might interfere with the spin-down. I'll test it and see.
An underlying question is whether leaving /var on the SD, shortens it's life. And if it does, will this be of practical significance to me or other users?