Since I am quite new to looking at lamba sensors I was quite surprised at the physical size of them !
It appears to me that most if not all probes are on an 18mm thread in a 25mm bung. In a 40mm exhaust that's quite a hole ! It's looks like they protrude to the midpoint of the tube so that's 16x20 = 320m^2 of 1256mm^2 or 25% of the cross sectional area of the exhaust blocked up !
The exhaust headers on my Cagiva are already fitted with bungs from factory which appear to be M10 or M12 (have only eyeballed them at the present) is there a smaller probe for such a bung ?

Please somebody tell me I have got it all wrong....

The factory bungs are not for sensors but instead external test gear. AFAIK 18mm is the standard size for sensors.

Going back to Pouakai's original point, does having this large O2 sensor protruding into one side of the stock x-over have an adverse effect?

Looking at an LC-1 probe it is certainly a large device which must restrict the flow and perhaps give a different reading, we then set our fuel flow to suit that side but may then have the wrong fuel flow for the other side. Is the stock position satisfactory?  I have the 2003 x-over from a lamda equiped bike rather than the old large one fitted to earlier models. I take Luhbo's point about the thing not looking too good if fitted to the downpipes but they could be mounted on the front tubes of the x-over under the gearbox using those bungs he mentioned in his link.
http://www.efi-parts.co.uk/index.php?productID=131

Still, the original question was about the restriction in the pipe - does anyone have a definitive answer?

Of course Pouakai is probably using the standard fitment point which is in the LH side of the x-over, the bung is almost flush so the sensor does stick a long way into the exhaust duct, like me I suspect he wants to know if it has any effect, restricting flow, changed back pressure etc which of course will only affect one side.
Does anyone know if it would be better to fit a blanking  plug to that location and weld on a longer threaded boss elsewhere so only the tip protrudes into the gas flow. Something like this - 

http://www.rallysportdirect.com/Innovate-Motorsports-Stainless-Steel-Extended-1i...

Is this worth doing or will it have no real benifit?

Thanks LOS, I have been using the standard Guzzi O2 sensor screwed into the existing port on the LH side of the x-over and it appears to work  (using Breva voltage settings).  I now have two second hand LC-1 units although only one functioning sensor at present - have to wait to next month to buy the second sensor. I had intended to fit the working LC-1 sensor to the bike and reset the map to the default LC-1 settings. Now the concern about the probe limiting the flow has been raised I am thinking of adding two more ports with deeper threaded sections to allow the probes to have minimal interference.

I like the diagram of the O2 sensor, must have taken some research to find. Sadly I cannot get your second link to work.

I think we have to be careful using Bernoulli in an exhaust duct, the velocity, temperature and density are constantly changing, add in supersonic shock waves etc.

I suppose empirical research and experience are our best guide.

Have other folk concerned themselves with this matter of the probe obstructing the exhaust pipe, did they modify things away from the original Guzzi design?

Perhaps this is the wrong forum for this discussion, I will wander over to those nice people at www.v11lemans.com under my other identity as 68C and ask the question there.