Question: Is it possible to re-use the nxt_db directory when upgrading from 0.7.4 to 0.7.5 without downloading the whole blockchain again?
Yes, it is designed to be possible to upgrade without having to delete the nxt_db directory, that's the reason I bother with applying those SQL statements.
If too slow for Raspberries, do whichever is faster, upgrade or download from scratch.
Did I manage to improve the Raspberry performance with the last release?
What exactly do you mean by performance?
1) startup is slow as always. Especially these migrations
2) running is as good as always. Does not go above 5%.
3) RAM consumption is 1% lower. Still 87% of total memory.