how do they know, did they have it running on the bench?
Forgive my natural critique, It's just that a mechanic can say a lot of things, and i've seen turbos get swapped without checking boost pipes even!
I quess as both garages was landrover specialist i assume they know what there on about mate ,Both garages checked all my boost pipes the last 1 even commented on my silicone replacements and stated how good they was compared to the prone to failure oe ones .
The 1st garage could not comment on the flat response through the turbo range (as he thought it was quick but lumpy like it needed retuning) but did comment on the high pitch tinny squeel ,The 2nd garage the machanic who looked at my car happens to have a zs with equal mods to mine so knew what to expect power wise and he instantly said it was down on power and as soon as he heard the tinny high pitched squeel from the turbo instantly stated it was the turbo.
Im not a landrover/rover/mg specialist so i cannot comment although just being realistic about this .
The turbo IS the only thing that has been changed since mine failed all pipes/intercooler has been cleaned/degreased all pipework is correct no leakages what so every as i myself have removed these and checked properly ( ihave worked on cars myself for over 15 years all be it petrols mainly).
The problems im having are only since the turbo has been fitted and as i suspected the turbo was at fault i went to enquire with these (NOT stating what i thought it was) but simply discribed problems/symtems to them in which they took a look then took it for a test drive and as i said mate i didnt tell them what i thought it was yet they both diagnosed with the same thing turbo fault