Underneath the car behind cat there is another.

When you look at the photo's of these they appear to only have one lambda point , is there two types maybe, an early one and a later revised model. Anyway it will be an 18 x 1.5 pitch they all are, these bungs are getting harder to find as umbrako who used to make them don't do them anymore. The one supplier that I know of got a hole heap made up but they were parallel instead of being tapered ( if not tapered they will wind all the way through into the exhaust and wont seal properly ). Anyway hopefully an exhaust shop will have these.

"I just checked I only got one oxy sensor" - do you mean after the cat I assume?, you should be able to see the wires and top of the first oxy sensor in the dump pipe, under the heatshield, near the fire wall .

If its just the cat changed, yes fine to drive straight to your tuners, just keep it off boost, 2000rpm max and you will be sweet.

Really only injectors changed you never want to drive with- best done at the tuners that one.

so just a cat change with the standard dump will still overboost ?

Yes.. Without a tune to suit..