[KorbenDallas]

I think 64k could be still to small (as 32k didn't work at all with indirect connections).

I did some more testing. 1024K and 512K are unusable - BD ISOs open very slowly.
256K is better but still slow with some ISOs (20+ seconds).
128K is better but much slower than 64K.

I get the best results with 64K. 

Thanks @smp1 for the test builds!

I tested more and agree with you that 64k chunk size works fine with direct (wired) connection. But from 64k to 128k there is a big improvement in terms of reduced stuttering for non-direct conncetions (the initial setup with 2048k worked flawlessly). 128k still has some glitches but ISO opening time is acceptible (ca. 15 secs). So I would vote for 256K - it prolongs the opening time (to my mind compared to stuttering in a still acceptible level hopefully) but increases the chance, that people can watch hibit media via SMP.

BTW: does the protocol level influence the overhead/net speed (i. e. is a SMP1 connection faster/slower than a SMP3 connection)? If so, I will test the fastest standard as this is selectable directly in KODI settings.

Any chance to make the chunk size selectable or distinguish chunk size for initial reading and media playing?

Cheers