Rigga,
I'm afraid I have to disagree - but gently, I hope.
The Navy's rule was 'train as you fight'. In engineering terms, that meant we were given the training and top cover to be able to execute service modifications in an operational scenario within the peacetime rules. All our modifications were approved at high level by a responsive and well trained Engineering Authority. It's a surprising fact that the RN, with its reputation for buccaneering and independent action, had a very tightly run engineering system. It worked during two wars. Damn big ones.
My experience of STFs was that once they were fitted (and they should have been SEMs) they were damned hard to take off. I could (but I won't here) name you several instances of RAF aircraft flying around for years with modifications fitted that were simply not fit for purpose. RAF aircrew were not willing to forgo any extra capabilities they may have offered, and RAF engineers just didn't have the clout to insist that they were taken off.
I have had multiple experiences of 'in theatre/operational/urgent' - choose your excuse - modifications being put on aircraft and causing significant risk to the aircrew (RN and RAF, by the way). This is the area that Tucumseh rightly identifies as 'just not on'. I agree with him.
But what I suppose I'm trying to say is that you can 'square the circle' between Tucumseh and Tourist - they are both right. You have to get the mods into service to meet the need, but you have to do the job properly first time to make sure that you know what is actually fitted to the aircraft and just what the hazards posed are. My view (unsurprisingly) is that engineers are the people who should run that part of the job, with aircrew support. It's also my view that in certain areas of the RAF, the engineers didn't have the necessary 'clout' to make sure that happened.
In my view, this was a systemic issue that, in part, contributed to the safety failings identified by the various ART reports, on to the Nimrod accident and on to Haddon-Cave.
Best Regards as ever
Enginesd