Ok to answer to myself :

Invalid shares was the result of either a faulty switch or IP conflict (all my switches being Netgear prosafe). Strange thing, if i add a new cpl plug i have the same colficting problem than whith the supposedly faulty switch …

In regards to Hashrate :
suprnova skein : its more around 13GH/s on average (calculated 2 hours)
zpool skein : i have the same result betwee 12,5 to 13,5 GH (2h with 5 x G28)

So far suprnova payout > by 20% of Zpool skein (paid in DGB), calcultaed on 24h with 5 G28 (not the one i used for tests so with stable hashrate). Luck difference being 5% in advantage to suprnova