Thanks all for your replies!

So we go out and buy some more disks. I forgot to mention but the temp file folder grows to more than 60GB.
And it must be fast disks as I/O is huge.

Will increasing proxy_buffers help in our case to reduce disk i/o?
- during cache population
- if cache is populatet
And i was wrong above, average file size served is 140MB, largest around 2GB.

regards
Andi