Have had many eternal car systems as good or better than GM. tWIN PIPE 3.0 BOX A DIRECT FIT. Not really a diy job if your old one been on for 10 yrs...
Rubbish!
If the downpipe to system bolts undo, then it's a 40minute start-to-finish job, on your own, in the street.
If using a DIY drill add 10minutes per bolt that doesn't undo. Replace with a longer bolt and nut. If you have oxy-acetylene, heat the stub of the bolt to cherry red and punch the bastard thing out - 1minute plus however long it takes to get the tools out.
This is a job where DIY not only makes sense, but is to be recommended.
Whilst I agree that DIY is nearly always the way to go, if those bolts shear, its rarely "10mins per bolt", only if you're really lucky.
The last one I did took 22 bits to drill just one of the bolts out (they other 2 drilled out much better)...
the bolts harden, due to the continued heating and cooling of the system, quality drill bits help.
my own personal way, I know nickw, will shout, I remove the downpipes to drill them out, as I don't have access to a blue spanner at home, but do have use of one at work if I really need too.