dragging this one up from the past a bit, but I think what I have to say is worthy of it.
We had our first 6 player, same room/location, online experience last night. Unfortunately the weather, holidays and work commitments meant this was our first opportunity since GT5 was released but I'm happy to report that it went very smoothly!
It seems like the lack of LAN play is not a major stumbling block, we had 6 PS3s connected to PSN and in the same lobby, racing against each other. The lag was barely noticeable (worse if someone spun out right in front of you). Obviously it wasn't at LAN levels of non-lag but perfectly acceptable and this is on a 'slow' connection compared to what is available out there.
Overall, we are very happy!