620s racers...have you?
 

Airboxes/crankcase breathers:

Have any of you disconnected the crankcase breather from the expansion tank, thrown the tank away and then blocked the connection from the airbox to the expansion tank?

Dumping warm oily air from a gearbox into an airbox is done for pollution and emission regs but, it's generally thought to be pretty bad practice as far as performance is concerned.
Back in the mid-80's my CBX550 had such an arrangement. When I removed the gubbins it gave an instant performance increase amounting to about 5mph at the top end. It made it a bugger to warm up though (knew nothing about jetting in those days).

Frames:
I am breaking on of my 800's for spares. It happens to have a bent frame where the forks have quite clearly hit the frame tubes as the steering turned forcibly back in a crash. I understand that this is a common cause of writing the bike off. The frame is a bit soft.
Anyway, on this particular example, although somehow the forks have contacted the frame tubes, the lockstops were not broken in anyway. It might be worth your while adjusting the stops to reduce the amount of steering lock you have to give that extra margin just in case. You also might want to consider putting some thick rubber pads on the frame to absorb any energy that might come from contact with the forks in a crash.

I had an idea that it was standard practice to vent the various breathers to atmosphere on a race bike. Well actually into a catchtank somewhere (plastic bottle, drinks can etc).

Not necessarily std. I think the rules only say that hoses must vent into catch tanks - the catch tank can be anything you want within reason including the airbox as originally designed.

I was wondering if anyone had noticed a change in the fuelling and performance by removing the vent return from the airbox.

Rules say something along the lines of "being easily removable for emptying" Ducati air box’s are anything but easy to remove. Talk to Skids, I think he has an opinion on this matter.