I think who people trust is very subjective, andy forrest has had some choice words to say about Duncan and personally i wouldn't trust him. But he's well aware of that
I've used clinic for years and always will, some of the job's i have seen that have come from other so called specialists have been abysmal. Do clinic make mistakes now and again as well? yes, there only human, but all it takes is a phone call or a quick visit to put right and they bend over backwards to sort it.
As far as needing a map after changing exhaust, bit dependant if its cat change or not. Prices always seem to be a hotly debated subject though, i do know what clinic charge for a map and why plus it is always a fixed amount, If it takes 2 or 5 hours to map is the same price, bear in mind clinic use a subcontract mapper to do most of their mapping and he has a set fee.
I hear story's now and again about 'overcharing' then when you find out what's actually happened it isn't as simple, typical one is go in for x and y,z and D are found to also be an issue, now is that over charging or rectifying genuine faults? I think you can say the same about allot of places in that sense as well.
You pays yer money you takes your choice :)
Although it is ironic that if you show loyalty now adays you get flak for it lol