What do I think?

and

In short, - impressive - and I wish I was visiting you to see how it all fits together. "1000" partitions is not something that happens everyday!
I am guessing that the highest numbers are in the VIOS - as they need the most slots, so I am curious to hear what the highest numbers are for the clients.
And I am wondering if you have created any space for both VSCSI and NPIV connections. There are reasons for having both.
If you have only NPIV right now then you will not be able to use either SSP (shared storage pools) nor PowerSC (Trusted Logging).
If you are planning to only use SSP - I can almost assure you that you will want NPIV for a few partitions.
Assuming this, and knowing you have 5 pairs of VIOS and 900 plus clients planned I would consider sufficient justification for a multiple models of VIOS (so, one model that is supporting only VSCSI, and one support only NPIV for storage plus limited VSCSI for security). I would think about how to have VSCSI in both (equivalent to equal) for supporting security (PowerSC trusted logging is the easiest example) but have the clients still "all the same".
re: VIOS support for PowerSC, another choice could be X (up to 5) additional VIOS, but only for security (no SEA, no NPIV, no SSP/VSCSI storage support - only security).
As far as the performance impact. I cannot honestly say. However, starting with the 730 firmware many changes were made in the firmware - one of them being supporting more than 254 partitions. So I would hope that many of the performance "impacts" of using large slot numbers have been addressed.
And I am sure that there are more "external" people who would love to come take a look.
Please excuse my drooling...
