I was fairly confident that it does take place server-side, but you've got me wondering. I didn't know you could even change gear while R0ing for example. Of course the server does perform the final check, if there's a client-side check involved, it would be to reduce server-spam, so that people don't try to equip items they can't wear all the time. Also, it would help the client change gear before the actual gear change takes place, if the change is valid, which makes it appear as reduced lag.

The amount of items changed doesn't matter so much, as the duration for sequencing the data that's being transferred is still minimal compared to the actual round-trip time (which is the major delimiting factor), and it does in fact cause lag sometimes, which skips certain gear changes. Although to be perfectly honest, I'm not sure if that's always due to this lag, or if there are other factors involved.

Guess I'll have to do some further testing when I get the chance to log in next time.